CHEK1 and cancer: To better understand the response of CDKN2A mutant or deleted cancer cell lines to PMX we next performed immunoblots of the proteins involved in the thymidine nucleotide metabolism pathway, including TYMS, Thymidine phosphorylase (TYMP), thymidine kinase 1 (TK1), glycinamide ribonucleotide formyltransferase (GART) and dihydrofolate reductase (DHFR) as well as cleaved PARP1, and p-CHK1 as markers of apoptosis and DNA damage checkpoint activation, respectively.