DOK4 (Downstream of kinase 4; also IRS5) is a 41 kDa member of the type B subfamily, DOK family of proteins. DOK4 is expressed in T cells and endothelium, plus embryonic neurons and renal anlage, and serves as an intermediary between the InsR and downstream MAP kinase activation. Human DOK4 is 326 amino acids (aa) in length. It contains an N-terminal insulin receptor substrate (IRS) pleckstrin homology domain (aa 7 - 109) and a centrally placed IRS phosphotyrosine-binding domain (aa 125 - 236). There is a potential alternate start site at Met16. Between aa 190 - 326, human DOK4 shares 96% and 98% aa identity with mouse and canine DOK4, respectively.