LAPTM4B and breast cancer: Furthermore, knockdown of LAPTM4b in two cancer cell lines known to overexpress LAPTM4b15, 16, the breast cancer MDA-MB-231 and the hepatic carcinoma HepG2 lines, which also express high levels of endogenous LAT1 (Supplementary Fig. 6a,b), led to a marked reduction in S6K1 phosphorylation (Supplementary Fig. 6c–f).