Because MUC1 is over-expressed by most human carcinomas, where it is associated with tumor progression and poor prognosis [22], we asked whether constitutive levels of MUC1 were linked to stable differences in intrinsic Δψm and, as a consequence, were a component or maker of tumor cell heterogeneity in colonic and/or mammary carcinoma cell populations. Here, MUC1 is linked to breast carcinoma.