Abstract BackgroundHER-3 has a role in pathogenesis and development of resistance to targeted therapies in non-small cell lung cancer (NSCLC). MethodsWe analysed tumour samples from 45 patients with adenocarcinoma of the lung. HER-3 and HER-2 expression was identified using immunohistochemistry as well as bioinformatic interrogation of The Cancer Genome Atlas (TGCA). ResultsHER-3 was highly expressed in 42.2% of cases and observed more frequently than HER-2 overexpression. ERBB3 copy number alone did not account for HER-3 overexpression. Bioinformatic analysis of TCGA demonstrated that MEK activity score (a score of downstream pathway activity and surrogate of functional HER-3 signalling) did not correlate with HER-3 ligands. EGFR pathway signalling was predominant with a significantly positive correlation of EGFR and AREG expression with MEK activity score. In the ERBB3 expressed group, ERBB3 RNA expression levels were significantly correlated with MEK activity score. ERBB3 expression level remained highly positively correlated with MEK activity after adjusting for EGFR expression. ConclusionHER-3 expression is common and it is not associated with gene amplification nor is it ligand dependent. HER-3 is an alternative pathway to EGFR activating MEK. HER-3 is a potential therapeutic target in NSCLC by virtue of frequent overexpression and functional downstream signalling.