Agglomerative hierarchical clustering according to similarity in gene expression patterns in cell lines harboring translocations that are common in ALL. For each cell line, polyA-purified RNA and common reference polyA RNA⁸  were fluorescently labeled (with Cy5- or Cy3-dUTP) and comparatively hybridized to an array containing approximately 24 000 elements representing approximately 19 000 genes (NCBI Unigene Build no. 160). The variation in expression is displayed as a variation in color¹⁰  for the 331 clones (306 genes) for which the log₂ intensity ratio differed by at least 2 from its mean on at least 2 arrays. The color scale extends from 0.25- to 4.0-fold of the mean (-2 to 2 in log₂ space) as indicated in the upper-right corner. Gray represents data that was omitted because it was not well measured as described in “Materials and methods.” The black bars indicate duplicate measurements of the same cell line. The arms of the dendrogram are color-coded to indicate the chromosomal translocation associated with each branch: purple indicates MLL; brown, t(17;19); orange, t(12;21); black, t(5;12); pink, t(1;19); and blue, t(9;22). The broad features of the clustering patterns were robust to variations in gene selection criteria.