ACK1 (Activated CDC42 kinase 1) is an enzyme that in humans is encoded by the TNK2 gene. TNK2 gene encodes a non-receptor tyrosine kinase, ACK1, that binds to multiple receptor tyrosine kinases e.g. EGFR, MERTK, AXL, HER2 and insulin receptor (IR). ACK1 also interacts with Cdc42Hs in its GTP-bound form and inhibits both the intrinsic and GTPase-activating protein (GAP)-stimulated GTPase activity of Cdc42Hs. This binding is mediated by a unique sequence of 47 amino acids C-terminal to an SH3 domain. The protein may be involved in a regulatory mechanism that sustains the GTP-bound active form of Cdc42Hs and which is directly linked to a tyrosine phosphorylation signal transduction pathway. CK1 or TNK2 has been shown to interact with AKT, Androgen receptor or AR, a tumor suppressor WWOX, FYN and Grb2. ACK1 interaction with its substrates resulted in their phosphorylation at specific tyrosine residues.