diff --git a/R/generate_matrices_ml.R b/R/generate_matrices_ml.R
index 978edb0..12b3cd3 100644
--- a/R/generate_matrices_ml.R
+++ b/R/generate_matrices_ml.R
@@ -883,6 +883,331 @@ skipImbalancedMatrix <- function(genome_ids,
   split
 }
 
+#' Build leave-one-out (LOO) merged parquet matrices from drug parquet files.
+#'
+#' @param path Character. Base directory containing stratified parquet matrices.
+#'             Expected subdirs: matrix/..
+#' @param verbosity Character. "minimal" or "debug"; when "debug", prints detailed steps.
+#' @return Invisibly returns a tibble with paths of created LOO parquet files.
+#' @import arrow dplyr purrr stringr tibble
+.parquet2LOODrugMatrix <- function(
+  path,
+  verbosity = c("minimal", "debug")
+) {
+  verbosity <- match.arg(verbosity)
+  log <- .make_logger(verbosity)
+
+  ## ------------------------------------------------------------------
+  ## Paths
+  ## ------------------------------------------------------------------
+  matrix_path <- gsub("\\\\", "/", file.path(path, "matrix"))
+  LOO_path <- gsub("\\\\", "/", file.path(path, "LOO_matrix_drug"))
+
+  if (!dir.exists(matrix_path)) {
+    log("info", paste0("Matrix directory does not exist: ", matrix_path))
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+  if (!dir.exists(LOO_path)) dir.create(LOO_path, recursive = TRUE)
+
+  ## ------------------------------------------------------------------
+  ## Locate parquet files
+  ## ------------------------------------------------------------------
+  files <- list.files(matrix_path, pattern = "\\.parquet$", full.names = TRUE)
+  files <- files[!grepl("drug_class", basename(files))]
+  files <- gsub("\\\\", "/", files)
+
+  if (length(files) == 0) {
+    log("info", paste0("No parquet files found in ", matrix_path))
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+
+  log("debug", paste0("Found ", length(files), " parquet files"))
+
+  ## ------------------------------------------------------------------
+  ## Parse filenames
+  ## Expected: <prefix>_<drug>_<feature>_sparse.parquet
+  ## ------------------------------------------------------------------
+  parsed_info <- tibble::tibble(
+    file  = files,
+    parts = stringr::str_split(basename(files), "_")
+  ) |>
+    dplyr::mutate(
+      idx_sparse = purrr::map_int(parts, \(x) {
+        w <- which(x == "sparse.parquet")
+        if (length(w) == 0) NA_integer_ else w[1]
+      }),
+      feature = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i) || i < 6) {
+          NA_character_
+        } else {
+          paste(x[(i - 2):(i - 1)], collapse = "_")
+        }
+      }),
+      prefix = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i) || i < 3) {
+          NA_character_
+        } else {
+          paste(x[1:(i - 4)], collapse = "_")
+        }
+      }),
+      drug = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i)) NA_character_ else x[i - 3]
+      })
+    ) |>
+    dplyr::filter(!is.na(prefix), !is.na(feature), !is.na(drug))
+
+  if (nrow(parsed_info) == 0) {
+    log("info", "No files matched expected naming pattern")
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+
+  log("debug", paste0("Parsed ", nrow(parsed_info), " files"))
+
+  ## ------------------------------------------------------------------
+  ## Iterate over (prefix, feature)
+  ## ------------------------------------------------------------------
+  prefix_feature <- parsed_info |>
+    dplyr::distinct(prefix, feature)
+
+  created <- character(0)
+
+  for (i in seq_len(nrow(prefix_feature))) {
+    sub_prefix <- prefix_feature$prefix[i]
+    sub_feature <- prefix_feature$feature[i]
+
+    group_df <- parsed_info |>
+      dplyr::filter(prefix == sub_prefix, feature == sub_feature)
+
+    if (nrow(group_df) == 0) next
+
+    ## Split by drug
+    grouped <- split(group_df, group_df$drug)
+
+    ## --------------------------------------------------------------
+    ## Leave‑one‑drug‑out with genome blocking
+    ## --------------------------------------------------------------
+    purrr::walk(names(grouped), function(leave_one_out) {
+      ## Read held‑out drug to get its genome IDs
+      heldout_tbl <- arrow::read_parquet(grouped[[leave_one_out]]$file)
+
+      if (!"genome_id" %in% colnames(heldout_tbl)) {
+        stop("Column '", "genome_id", "' not found in parquet file")
+      }
+
+      heldout_genomes <- unique(heldout_tbl[["genome_id"]])
+
+      ## Training drugs
+      subset <- grouped[names(grouped) != leave_one_out]
+
+      ## Remove overlapping genomes (STRICT leakage control)
+      subset <- purrr::map(
+        subset,
+        \(df) {
+          tbl <- arrow::read_parquet(df$file)
+          dplyr::filter(tbl, !.data[["genome_id"]] %in% heldout_genomes)
+        }
+      )
+
+      ## Drop empty datasets
+      subset <- subset[purrr::map_int(subset, nrow) > 0]
+
+      if (length(subset) == 0) {
+        log(
+          "debug",
+          paste0(
+            "Skipping LOO drug ", leave_one_out,
+            ": no data left after genome filtering"
+          )
+        )
+        return(NULL)
+      }
+
+      combined <- dplyr::bind_rows(subset)
+
+      ## Safety check (can comment out once stable)
+      stopifnot(!any(combined[["genome_id"]] %in% heldout_genomes))
+
+      out_file <- gsub("\\\\", "/", file.path(
+        LOO_path,
+        paste0(
+          sub_prefix, "_leaveout_", leave_one_out, "_",
+          sub_feature, "_sparse.parquet"
+        )
+      ))
+
+      arrow::write_parquet(combined, out_file)
+      created <<- c(created, out_file)
+
+      log(
+        "debug",
+        paste0(
+          "Created LOO file: ", out_file,
+          " | removed ", length(heldout_genomes), " genomes"
+        )
+      )
+    })
+  }
+
+  log("info", "All LOO‑drug matrices generated with genome‑level blocking")
+  invisible(tibble::tibble(created_file = created))
+}
+
+.parquet2CrossDrugTestMatrix <- function(
+  path,
+  verbosity = c("minimal", "debug")
+) {
+  verbosity <- match.arg(verbosity)
+  log <- .make_logger(verbosity)
+
+  ## ------------------------------------------------------------------
+  ## Paths
+  ## ------------------------------------------------------------------
+  matrix_path <- gsub("\\\\", "/", file.path(path, "matrix"))
+  out_path <- gsub("\\\\", "/", file.path(path, "cross_drug_test"))
+
+  if (!dir.exists(matrix_path)) {
+    log("info", paste0("Matrix directory does not exist: ", matrix_path))
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+  if (!dir.exists(out_path)) dir.create(out_path, recursive = TRUE)
+
+  ## ------------------------------------------------------------------
+  ## Locate parquet files
+  ## ------------------------------------------------------------------
+  files <- list.files(matrix_path, pattern = "\\.parquet$", full.names = TRUE)
+  files <- files[!grepl("drug_class", basename(files))]
+  files <- gsub("\\\\", "/", files)
+
+  if (length(files) == 0) {
+    log("info", paste0("No parquet files found in ", matrix_path))
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+
+  log("debug", paste0("Found ", length(files), " parquet files"))
+
+  ## ------------------------------------------------------------------
+  ## Parse filenames (same logic as LOO)
+  ## ------------------------------------------------------------------
+  parsed_info <- tibble::tibble(
+    file  = files,
+    parts = stringr::str_split(basename(files), "_")
+  ) |>
+    dplyr::mutate(
+      idx_sparse = purrr::map_int(parts, \(x) {
+        w <- which(x == "sparse.parquet")
+        if (length(w) == 0) NA_integer_ else w[1]
+      }),
+      feature = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i) || i < 6) {
+          NA_character_
+        } else {
+          paste(x[(i - 2):(i - 1)], collapse = "_")
+        }
+      }),
+      prefix = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i) || i < 3) {
+          NA_character_
+        } else {
+          paste(x[1:(i - 4)], collapse = "_")
+        }
+      }),
+      drug = purrr::pmap_chr(list(parts, idx_sparse), \(x, i) {
+        if (is.na(i)) NA_character_ else x[i - 3]
+      })
+    ) |>
+    dplyr::filter(!is.na(prefix), !is.na(feature), !is.na(drug))
+
+  if (nrow(parsed_info) == 0) {
+    log("info", "No files matched expected naming pattern")
+    return(invisible(tibble::tibble(created_file = character())))
+  }
+
+  log("debug", paste0("Parsed ", nrow(parsed_info), " files"))
+
+  ## ------------------------------------------------------------------
+  ## Iterate over (prefix, feature)
+  ## ------------------------------------------------------------------
+  prefix_feature <- parsed_info |>
+    dplyr::distinct(prefix, feature)
+
+  created <- character(0)
+
+  for (i in seq_len(nrow(prefix_feature))) {
+    sub_prefix <- prefix_feature$prefix[i]
+    sub_feature <- prefix_feature$feature[i]
+
+    group_df <- parsed_info |>
+      dplyr::filter(prefix == sub_prefix, feature == sub_feature)
+
+    if (nrow(group_df) < 2) next
+
+    grouped <- split(group_df, group_df$drug)
+
+    drugs <- names(grouped)
+
+    ## --------------------------------------------------------------
+    ## Pairwise cross‑drug testing (A → B)
+    ## --------------------------------------------------------------
+    for (drugA in drugs) {
+      for (drugB in drugs) {
+        if (drugA == drugB) next
+
+        ## Read training (drug A)
+        train_tbl <- arrow::read_parquet(grouped[[drugA]]$file)
+
+        if (!"genome_id" %in% colnames(train_tbl)) {
+          stop("Column '", "genome_id", "' not found in parquet file")
+        }
+
+        train_genomes <- unique(train_tbl[["genome_id"]])
+
+        ## Read test (drug B) and REMOVE overlapping genomes
+        test_tbl <- arrow::read_parquet(grouped[[drugB]]$file) |>
+          dplyr::filter(!.data[["genome_id"]] %in% train_genomes)
+
+        if (nrow(test_tbl) == 0) {
+          log(
+            "debug",
+            paste0(
+              "Skipping ", drugA, " → ", drugB,
+              ": no test data after genome filtering"
+            )
+          )
+          next
+        }
+
+        ## Safety check
+        stopifnot(!any(test_tbl[["genome_id"]] %in% train_genomes))
+
+        out_file <- gsub("\\\\", "/", file.path(
+          out_path,
+          paste0(
+            sub_prefix, "_", drugA,
+            "_cross_testing_", drugB, "_",
+            sub_feature, "_sparse.parquet"
+          )
+        ))
+
+        arrow::write_parquet(test_tbl, out_file)
+        created <- c(created, out_file)
+
+        log(
+          "debug",
+          paste0(
+            "Created cross‑drug test: ",
+            drugA, " → ", drugB,
+            " | removed ", length(train_genomes), " genomes"
+          )
+        )
+      }
+    }
+  }
+
+  log("info", "All cross‑drug testing matrices generated")
+  invisible(tibble::tibble(created_file = created))
+}
+
+
 #' Generate all ML input matrices
 #'
 #' a) each drug/drugclass matrices for all data and feature types
@@ -1022,6 +1347,11 @@ generateMLInputs <- function(parquet_duckdb_path = "results/Cje_parquet.duckdb",
     verbosity = verbosity
   )
 
+  .parquet2CrossDrugTestMatrix(out_path, verbosity = verbosity)
+
+  .parquet2LOODrugMatrix(out_path, verbosity = verbosity)
+
+
   log("info", "All matrices generated and saved.")
   invisible(TRUE)
 }