Mtb does indeed possessboth de novo and salvage, but Griffin et al. usedrandom transposon mutagenesis to identify that the hpt gene is essential for growth.22 In addition,human HGPRT is also being recognized more widely as a target for drugdiscovery against human cancers.23−26 Taken together, these studiesdemonstrate the crucial role that HG(X)PRT activity plays in almostall organisms. This evidence concerns the gene HPRT1 and cancer.