Given that HER3 is a kinase-inactive protein and requires a dimerisation partner to become phosphorylated (Guy et al, 1994; Kim et al, 1998; Holbro et al, 2003), our data indicate that the kinase activity of MET is required for formation of the MET–HER3 heterodimer, which results in the phosphorylation of HER3, in lung cancer cells with MET amplification, although it remains unclear whether HER3 phosphorylation is mediated by MET directly or by another kinase that is activated by MET. This evidence concerns the gene ERBB3 and lung carcinoma.