Here, we have found evidence that a single alteration event in MAGI3, caused by the utilization of a cryptic intron 10 PAS by premature cleavage and polyadenylation, occurs in approximately 7.5% of 160 breast cancers analyzed, and the resulting truncated MAGI3 product contributes to the malignant transformation of human mammary epithelial cells. Here, MAGI3 is linked to breast carcinoma.