MOS is a proto oncogene (c-Mos) encoded protein serine/threonine kinase. MOS is a monomeric protein that indirectly activates MAP kinase (Erk1/2) by directly phosphorylating MAP kinase kinase (Mck, MAPKK, MKK). MOS is known as a cytostatic factor (CSF) and is also thought to arrest unfertilized amphibian and mammalian cells during M phase, thus regulating oocyte maturation. MOS is destroyed before fertilisation, after exit from meiosis II, making it a good marker for studies of eggs during oogenesis and maturation.