The mucin (MUC) family consists of secreted and membrane-bound forms. The transmembrane mucin 1 (MUC1) is a heterodimer that is aberrantly overexpressed by diverse human carcinomas and certain hematologic malignancies. The MUC1 N-terminal (MUC1-N) and C-terminal (MUC1-C) subunits are generated by autocleavage within a SEA domain. The MUC1 cytoplasmic domain (MUC1-CD) located downstream of the SEA domain is sufficient for the induction of anchorage-independent growth and tumorigenicity; however, no information is available regarding the origin of these transforming sequences. Previous work demonstrated that, except for the SEA domain, MUC1 has no sequence homology with other membrane-bound mucins. The present results demonstrate that MUC1-CD evolved from repeat regions in the MUC5B secreted mucin. We also show that MUC1 sequences upstream to the SEA domain emerged from MUC5B. These findings indicate that both the MUC1-N and MUC1-C subunits evolved from secreted gel-forming mucins and that the MUC1-CD oncogenic function emerged by diversification after evolution from MUC5B.