TGFBR1 and breast cancer: Together with the observation that SB431542 inhibited cell growth with up-regulation of p21WAF1 expression in MDA-MB-231 and other non-BC cell lines [30], we arrived at the conclusion that ALK5-mediated activation of MEK-ERK signaling accounts, at least in part, for fueling and sustaining the ERK-aTGFβ1 regulatory loop (Figure 7).