Lymphocyte antigen 75 (Ly75/CD205) is a surface marker on mitotic germ cells in rainbow trout. In mammals, several cell surface molecular markers have been characterized in order to identify the mitotic germ cells. he expression profile of Ly75 protein was similar to that of the mRNA. Furthermore, identification of various fish homologs of ly75 confirmed that their amino acid sequences are well conserved. Therefore, Ly75 may be appropriate for use as a versatile cell surface marker for mitotic germ cells in fish.