Citron (rho-interacting, serine/threonine kinase 21) is an alias for the gene, Citron rho-interacting serine/threonine kinase, in humans. Other names include: Citron Rho-interacting kinase; CRIK; Serine/threonine-protein kinase 21; serine/threonine kinase 21; STK21. The encoded protein is an enzyme with an amino acid length of 2027 and a mass of 231.4 kDa. Other isoforms may exist. Citron (rho-interacting, serine/threonine kinase 21) is a member of the Protein kinase superfamily, AGC Ser/Thr protein kinase family. This gene shares homology with other species, including: Frog.