Hormone binding to the receptor initiates a series of events leading to phosphorylation of specific substrate proteins, which mediate the cellular responses such as changes in the activity of metabolic enzymes, gene expression, and cytoskeletal structures. The kinase cascade entails sequential activation of specific protein kinases induced by a signal from activated Ras protein. Second messengers (SM) play a role in some RTK signaling pathways, although not in the pathway depicted here. Likewise, some GPCR pathways do not involve second messengers; these lead to activation of MAP kinase. See text for discussion.