ERBB3 and breast cancer: A previous report demonstrated that cellular dependency on PI3K or MAPK signaling in HER2+ breast cancer could be predicted by RNA and protein expression levels of EGFR, ERBB3/HER3, and CDKN1B/p27.43 We therefore assessed associations between MPAS and EGFR, ERBB3, or CDKN1B transcript levels (Supplementary Figure 4).