SUMMARY Oligomerization of the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is important for optimal ligand binding and internalization. M6P/IGF2R is a tumor suppressor gene that exhibits loss of heterozygosity and is mutated in several cancers. We tested the potential dominant-negative effects of two cancer-associated mutations that truncate the M6P/IGF2R in ectodomain repeats 9 and 14. Our hypothesis was that co-expression of the truncated receptors with wild-type/endogenous, full-length M6P/IGF2R would interfere with M6P/IGF2R function by heterodimer interference. Immunoprecipitation confirmed formation of heterodimeric complexes between full-length M6P/IGF2Rs and the truncated receptors, termed Rep9F and Rep14F. Remarkably, increasing expression of either Rep9F or Rep14F provoked decreased levels of full-length M6P/IGF2Rs in both cell lysates and plasma membranes, indicating a dominant-negative effect on receptor availability. Loss of full-length M6P/IGF2R was not due to increased proteasomal or lysosomal degradation, but instead arose from increased proteolytic cleavage of cell-surface M6P/IGF2Rs resulting in ectodomain release, by a mechanism that was inhibited by metal ion chelators. These data suggest that M6P/IGF2R truncation mutants may contribute to the cancer phenotype by decreasing availability of full-length M6P/IG2Rs to perform tumor-suppressive functions such as binding/internalization of receptor ligands like IGF-II.