Serine/threonine-protein kinase that acts as a regulatory link between the membrane-associated Ras GTPases and the MAPK/ERK cascade, and this critical regulatory link functions as a switch determining cell fate decisions. RAF1 activation initiates a mitogen-activated protein kinase (MAPK) cascade that comprises a sequential phosphorylation of the dual-specific MAPK kinases (MAP2K1/MEK1 and MAP2K2/MEK2) and the extracellular signal-regulated kinases (MAPK3/ERK1 and MAPK1/ERK2) (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Isoform 1 was present in all tissues tested: skeletal muscle, intestine, brain, gizzard, heart, lung, kidney, bone marrow, spleen and bursa of Fabricius. Isoform 2 was only detected in brain, heart and skeletal muscle. In brain and heart isoform 1 is more abundant than isoform 2. In skeletal muscle isoform 2 is more abundant than isoform 1. Phosphorylation at Ser-259 inactivates kinase activity. Dephosphorylation of Ser-259 by a complex containing protein phosphatase 1 relieves inactivation, leading to stimulate RAF1 activity (By similarity). Belongs to the protein kinase superfamily. TKL Ser/Thr protein kinase family. RAF subfamily.