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Autophosphorylation is phosphorylation of a protein kinase by its own catalytic activity or that of another member of the same protein kinase assembly. Distinguish modification within one chain from trans-autophosphorylation between partners.

In trans-autophosphorylation, one protein kinase chain phosphorylates another partner chain. “Auto” therefore does not require the modified site and catalytic active site to be on the same chain. The insulin receptor provides an activation-loop example; EGFR shows why autophosphorylation and catalytic activation need not be the same event.