A Fox12 cDNA was cloned from the Nile tilapia ovary by RT-PCR and subsequent RACE. Alignment of known Fox12 sequences from vertebrates confirmed the conservation of the Fox12 open reading frame and protein sequences, especially the forkhead domain and C-terminal region, while some homopolymeric runs of amino acids are found only in mammals but not in non-mammalian vertebrates. RT-PCR revealed that Fox12 is expressed in the tilapia brain (B), pituitary (P), gill, and gonads (G), with the highest level of expression in the ovary, reflecting the involvement of Fox12 in B-P-G axis. Northern blotting and in situ hybridization also revealed an evident sexual dimorphic expression pattern in the gonads. Fox12 mRNA was mainly detected in the granulosa cells surrounding the oocytes. The ovarian expression of Fox12 in tilapia begins early during the differentiation of the gonads and persists until adulthood, implying the involvement of Fox12 in fish gonad differentiation and the maintenance of ovarian function. (C) 2004 Elsevier Inc. All rights reserved.