MSH6 and drug resistance. (A) DNA copy number at 2p16.3, as inferred by SNP array. Each column represents a sample (orange indicates diagnosis; green, relapse; from left to right: patients 1-20). (B) Relative DNA copy number loss and reduced RNA expression of MSH6 in the 2 diagnosis/relapse pairs were confirmed by real-time PCR. (C) MSH6 gene expression and mercaptopurine sensitivity in primary ALL samples (n = 66). Samples with LC₅₀ more than or less than 2 mM were considered as resistant (n = 11) or sensitive (n = 55), respectively. (D) MSH6 gene expression and prednisone sensitivity in primary ALL samples (n = 51). Samples with LC₅₀ more than or less than 4 μM were considered as resistant (n = 17) or sensitive (n = 34), respectively. Boxes include data between the 25th and 75th percentiles, and whiskers indicate the minimal and maximal values excluding the outliers.