Bamacan can occur in certain cell types as either a secreted proteoglycan assembled into basement membranes or as an intracellular protein known as structural maintenance of chromosome 3 (SMC3). To assess the role of this protein in tumorigenesis, we investigated whether induced overexpression of bamacan/SMC3 could transform normal fibroblasts. We generated a full-length cDNA encoding the entire mouse bamacan/SMC3 and demonstrated appropriate transcription and translation into a 146-kDa protein. All the NIH and Balb/c 3T3 murine fibroblasts overexpressing this bamacan/SMC3 transgene generated foci of transformation and acquired anchorage-independent growth. The increased levels of bamacan/SMC3 expression achieved in the transfected fibroblasts were the same as those detected in a series of spontaneously transformed murine and human colon carcinoma cells. Moreover, a 3-4-fold overexpression of bamacan/SMC3 was detected in approximately 70% of human colon carcinoma specimens from matched pairs (n = 19, p < 0.0002) and in a cohort of intestinal tumors from Apc-deficient Min/+ mice. These results support the concept that deregulated expression of bamacan/SMC3 is involved in cell transformation.