DCK and UCK enzymes predict drug sensitivity. In cell lines representative of the histologic and genetic diversity of cancer (NCI60), the rate-limiting enzymes that determine the intracellular half-lives of decitabine and 5-azacytidine, DCK and UCK respectively, predicted drug sensitivity. Drug sensitivity is represented as GI₅₀: the drug concentration causing 50% growth reduction. The higher the GI₅₀, the less sensitive to drug, quantified by measurement of total protein at day 6 (raw data downloaded from the National Cancer Institute [http://dtp.nci.nih.gov] and GSE5846, gene expression by microarray). (A) Decitabine GI₅₀ is inversely correlated with DCK expression and growth fraction (higher DCK implies longer intracellular half-life and a higher doubling time implies a smaller S-phase fraction). Decitabine GI₅₀ did not correlate with UCK2 expression (data not shown). (B) 5-azacytidine GI₅₀ inversely correlates with UCK2 expression and growth fraction. 5-Azacytidine GI₅₀ did not correlate with DCK expression (data not shown). UCK1 expression was not available in this database.