Although experimental studies suggest that aberrant Ras function can promote the malignant progression of human breast epithelial cells, the occurrence of mutated ras genes in breast tumors is infrequent. One possible explanation for this apparent paradox is that aberrant function of the Ras-related protein TC21/R-Ras2, which causes malignant transformation of NIH 3T3 cells via upregulation of the Ras signal transduction pathway, may contribute to breast tumor development in the absence of Ras mutations. To address this possibility, we utilized two complementary approaches. First, we determined that aberrant TC21 function caused transformation of the MCF-10A human breast epithelial cell line. TC21-transformed MCF-10A cells exhibited altered cellular morphology associated with a disruption of cell-cell adherens junctions, formed colonies in soft agar, and showed enhanced motility in vitro. These alterations were similar to, but more dramatic than, those observed with oncogenic Ras-transformed MCF-10A cells. Furthermore, overexpression of normal TC21, but not Ras, also caused transformation of these cells. Second, we observed that TC21 protein expression was greatly elevated in 7 of 9 breast tumor lines when compared to untransformed MCF-10A cells. Taken together, these results support the possibility that overexpression of TC21 may contribute to aberrant growth properties of breast carcinoma cells.