We have isolated a rat complementary DNA clone corresponding to a novel isoform of the hormone nuclear receptor superfamily. This clone encodes a 383 amino acid residue protein designated as Rev-erb beta 2. This protein is identical until residue 382 to the Rev-erb beta 1 protein, which is 195 amino acids longer. Several arguments pointed out that the Rev-erb beta 2 cDNA may originate from the same gene as Rev-erb beta 1 by alternative splicing and using a different polyadenylation site. Our results indicate that Rev-erb beta 2 is a new isoform of the Rev-erb family of orphan nuclear receptors.