TP53 and neoplasm: MDM2 SNP309 (rs2279744; T/G) is located 309 base pairs downstream from intron 1 in the promoter of MDM2. The single nucleotide polymorphism (SNP) 309 T>G change has been found to enhance the affinity of the transcriptional activator Sp1, leading to increased levels of MDM2, and thereby weakening the p53 pathway of tumor suppression [10].