Genes expressed during spermatogenesis undergo alternative initiation and alternative splicing and may be under the control of a coordinated mechanism of RNA processing. A family of proteins that combine features of signal-transduction and RNA-binding molecules could be instrumental in this process. We have characterized a cDNA from adult chicken testis that codifies a highly conserved member of the STAR protein family, the orthologue of the mouse quaking gene qki. The predicted chicken protein differs only in four amino acids from the corresponding mouse protein. Messages of 7, 6, and 5 kb are expressed differentially during chicken spermatogenesis. The 5-kb message, the predominant form in adult testis, presents heterogeneity in the coding region, showing insertions of 51 and 75 bp and a deletion of 24 bp, which gives rise to four possible isoforms of the protein.