PRTFDC1 (phosphoribosyl transferase domain containing 1) is a phosphoribosyltransferase homolog with minimal catalytic activity toward purine bases. Structurally, PRTFDC1 resembles hypoxanthine-guanine phosphoribosyltransferase (HPRT) and binds GMP, IMP, and PRPP; however, its catalytic efficiency toward hypoxanthine and guanine is only 0.09-0.26% that of HPRT, primarily due to a glycine substitution at the catalytic aspartate position 1. Consequently, PRTFDC1 does not significantly contribute to purine salvage metabolism 1. PRTFDC1 has evolved under distinct evolutionary pressure compared to HPRT1, and was recently inactivated in mice but remains functional in humans 2. In disease contexts, PRTFDC1 appears to function as a tumor suppressor; its expression is frequently silenced through promoter hypermethylation in oral squamous-cell carcinomas 3 and ovarian cancers 4, and restoration of PRTFDC1 expression inhibits cancer cell growth 3. Genome-wide association studies identified PRTFDC1 as a significant locus associated with post-traumatic stress disorder (PTSD) 5 and implicated it in Sjögren's syndrome-related brain structural changes 6. In HPRT-deficient mice, human PRTFDC1 expression exacerbates aggressive behavior, suggesting genetic modulation of neurological phenotypes 7.