PLK4 and small cell osteogenic sarcoma: In clinical observations, a heterozygous 13‐bp deletion in the serine/threonine kinase domain of PLK4 was identified in azoospermia cases with Sertoli cell‐only syndrome (SCOS),34 and a heterozygous PLK4 mutation (p.Ile242Asn) in mice caused male germ cell loss in the testes,35 similar to that in human SCOS.