BT474 late stage lapatinib-resistant cells overexpress HER2 and HER ligands. (A) mRNA expression levels of HER receptors and ligands in BT474 parental and distinct resistant derivatives by qRT-PCR. Data were normalized to parental cells. (B) EGFR, HER2, and HER3 protein levels in BT474 parental, early, and late stage lapatinib-resistant cells. Protein level was quantified with Odyssey software (LI-COR Biosciences, Inc., Lincoln, NE). Each expression level was acquired from three independent samples for each derivative. Significance between groups was determined by multiple comparisons using the Sidak method (*P < 0.0001, EGFR expression: P versus LLR; HER3 expression: P versus LR, P versus LLR, or LR versus LLR).