Serine/threonine kinase which acts as an essential component of the MAP kinase signal transduction pathway. MAPK1/ERK2 and MAPK3/ERK1 are the 2 MAPKs which play an important role in the MAPK/ERK cascade. They participate also in a signaling cascade initiated by activated KIT and KITLG/SCF. Depending on the cellular context; the MAPK/ERK cascade mediates diverse biological functions such as cell growth; adhesion; survival and differentiation through the regulation of transcription; translation; cytoskeletal rearrangements. The MAPK/ERK cascade plays also a role in initiation and regulation of meiosis; mitosis; and postmitotic functions in differentiated cells by phosphorylating a number of transcription factors. About 160 substrates have already been discovered for ERKs. Many of these substrates are localized in the nucleus; and seem to participate in the regulation of transcription upon stimulation. However; other substrates are found in the cytosol as well as in other cellular organelles; and those are responsible for processes such as translation; mitosis and apoptosis. Moreover; the MAPK/ERK cascade is also involved in the regulation of the endosomal dynamics; including lysosome processing and endosome cycling through the perinuclear recycling compartment (PNRC); as well as in the fragmentation of the Golgi apparatus during mitosis. The substrates include transcription factors (such as ATF2; BCL6; ELK1; ERF; FOS; HSF4 or SPZ1); cytoskeletal elements (such as CANX; CTTN; GJA1; MAP2; MAPT; PXN; SORBS3 or STMN1); regulators of apoptosis (such as BAD; BTG2; CASP9; DAPK1; IER3; MCL1 or PPARG); regulators of translation (such as EIF4EBP1) and a variety of other signaling-related molecules (like ARHGEF2; FRS2 or GRB10). Protein kinases (such as RAF1; RPS6KA1/RSK1; RPS6KA3/RSK2; RPS6KA2/RSK3; RPS6KA6/RSK4; SYK; MKNK1/MNK1; MKNK2/MNK2; RPS6KA5/MSK1; RPS6KA4/MSK2; MAPKAPK3 or MAPKAPK5) and phosphatases (such as DUSP1; DUSP4; DUSP6 or DUSP16) are other substrates which enable the propagation the MAPK/ERK signal to additional cytosolic and nuclear targets; thereby extending the specificity of the cascade. {ECO:0000269|PubMed:10393181; ECO:0000269|PubMed:10617468; ECO:0000269|PubMed:12110590; ECO:0000269|PubMed:12356731; ECO:0000269|PubMed:12974390; ECO:0000269|PubMed:15788397; ECO:0000269|PubMed:15952796; ECO:0000269|PubMed:16581800; ECO:0000269|PubMed:19265199; ECO:0000269|PubMed:8325880; ECO:0000269|PubMed:9155018; ECO:0000269|PubMed:9480836}.