NRG1 and head and neck squamous cell carcinoma: Based on an analysis of NRG1 and ErbB3 mRNA in recurrent HNSCC, post-treatment, late stage disease also appears to be enriched for NRG1-mediated ErbB3 activation [40]; whether Trop2 loss is a mechanism for autocrine activation of ErbB3 in this context remains to be evaluated.