Eleven cDNA clones of retroviral origin (SORO) have been isolated and characterized from a rat cardiac cDNA library that are highly homologous to the RAL-element family of rat retroviral endogenous sequences. SORO-1, the largest of the clones isolated at approximately 3.5 kb, contains a characteristic long terminal repeat (LTR) that is unique to previously identified retroposons. Its LTR contains elements homologous to the CAAT box, a TATA box, a core enhancer sequence, and potential binding sites for GATA. SORO-1 hybridizes to a mRNA of approximately 7 kb, that is present both in rat heart and liver. This transcript was not, however, detected in other tissues (fast and slow skeletal muscles, brain, kidney, testis, lung, intestine, spleen) or from any other species (man, monkey, mouse, dog, rabbit, chicken, cow, yeast) examined. During postnatal cardiac development, the expression of the transcript appears to be down-regulated and is present at levels 10-fold greater in neonates than in adults. The function of this retroposon that is expressed in rat heart and liver has not yet been determined, but the sequence analysis of its LTR and its expression pattern suggest that it may be regulated by specific trans-acting factors that are present in both rat heart and liver.