The varying expression and subcellular localization of OCT4 both in the normal lactating breast and in the examined breast tumors together with the relatively small proportion of highly positive cells in the nucleus in all cases suggest that OCT4 is differentially expressed along a differentiation continuum of the mammary cellular hierarchy, with distinct expression levels and subcellular localization in different cell developmental states, and strongest expression in MaSCs. This evidence concerns the gene POU5F1 and breast neoplasm.