The expression levels of nuclear Six1 and levels of ERK phosphorylation (both nuclear and cytoplasmic pERK, intensity times percent of area) were significantly correlated (P < 0.05), These findings demonstrate that Six1 correlates with pERK in human breast cancer, and suggest that activation of ERK by Six1 may lead to expansion of TICs and to increased tumor aggressiveness. Here, SIX1 is linked to breast cancer.