2008), a murine cell‐surface mucin identified in mouse mammary carcinoma TA3‐Ha cells (Codington et al. 1972, 1975). TA3‐Ha cells are capable of allogeneic growth (Friberg 1972), proposed to result from epiglycanin preventing detection by immune cells (summarized by Itoh et al. 2008). This intriguingly suggests the potential of tumor protective properties of human MUC21 knockouts (e.g., the homozygous c.665delC subjects presented here). Here, MUC21 is linked to breast carcinoma.