Proteins encoded by the mdm2 gene, which has a pivotal role in the regulation of growth and differentiation, exist principally in human and murine cells as two isoforms that migrate in gels as 75-kDa and 90-kDa proteins. There is limited understanding of the respective biological roles of these isoforms, their molecular nature, and their mechanism of formation. We report here that human p75(MDM2) is an N-terminally truncated mixture of protein isoforms produced by the initiation of translation at two distinct internal AUG codons. The p75(MDM2) doublets and p90(MDM2), which is the full-length MDM2 protein, are expressed in approximately equal amounts from transcripts initiated at the constitutive P1 promoter of mdm2. Unlike murine transcripts initiated at the p53-activated P2 promoter, human cell transcripts initiated at the P2 promoter preferentially produce p90(MDM2). The ubiquitin enzyme variant protein TSG101, which interacts functionally with MDM2 in an autoregulatory loop that parallels the p53/MDM2 feedback control loop, interferes with degradation of both isoforms; however, only p90(MDM2) promotes proteolysis of TSG101 and p53. Our results reveal the mechanism of formation of the principal MDM2 isoforms, the differential effects of p53 on the production of these isoforms, and the differential abilities of human MDM2 isoforms as regulators of the MDM2/TSG101 and p53/MDM2 feedback control loops.