TSG101 is a tumor susceptibility gene whose homozygous functional knock out in fibroblasts leads to transformation and the ability of these cells to form metastatic tumors in nude mice. The cellular transformation that results from inactivation of TSG101 is reversible by restoration of TSG101 function. Decreased expression of TSG101 is associated with the occurrence of certain human cancers, including breast carcinomas. The TSG101 nucleic acid compositions find use in identifying homologous or related proteins and the DNA sequences encoding such proteins; in producing compositions that modulate the expression or function of the protein; and in studying associated physiological pathways. In addition, modulation of the gene activity in vivo is used for prophylactic and therapeutic purposes, such as treatment of cancer, identification of cell type based on expression, and the like. The DNA is further used as a diagnostic for a genetic predisposition to cancer, and to identify specific cancers having mutations in this gene.