Sun4, or Sperm associated antigen 4, is a human gene encoded by SPAG4. It may also be known as: Sperm-associated antigen 4 protein; SUN domain-containing protein 4; Outer dense fiber-associated protein SPAG4; acrosomal protein ACR55; Sad1 and UNC84 domain containing 4; cancer/testis antigen 127; CT127; SUN4. The encoded protein has an amino acid length of 437 and a mass of 48.2 kDa. Other isoforms may exist. Homologs of this gene exist in other organisms, including: Fly, Mouse, Rat, Chicken.