Estrogen receptor-related receptors (ERRs) are orphan members of the nuclear receptor superfamily, closely related to the estrogen receptor. With a PCR-based cloning strategy we have identified two cDNA isoforms from a neonatal mouse whole brain cDNA library encoding ERR gamma. We show that alternative splicing gives rise to different 5'-ends. The predicted peptide sequence of ERR gamma 2 differs from ERR gamma 1 by additional 23 N-terminal residues. The presence of identical isoforms in human suggests a pivotal function of ERR gamma in mammalia. Northern blot analysis reveals that ERR gamma is expressed as early as day 11 of embryonic development (E11). Whole mount in situ hybridization analysis shows major expression of ERR gamma in the central nervous system at E12.5. in the adult mouse a 5.7 kb transcript is detected in heart, brain, kidney, and skeletal muscle. Therefore, ERR gamma may be involved in the differentiation, as well as the maintenance of the differentiated properties in the brain.