We describe a polyadenylated RNA (Drld = Differentially regulated in lymphoid organs and differentiation) from murine lymphocytes. Two homologous molecular forms of this RNA of approximately 500 and 900 bases are detected in various tissues and lymphoid cell lines. Southern blotting showed that up to ten highly homologous loci exist which in part are polymorphic between the different mouse strains analyzed. Cross-hybridizing loci can be detected in distantly related species such as horse, fish and Drosophila melanogaster. Sequence analysis revealed homologies to the untranslated region of the GPI-linked murine B7(2) antigen, the B2 repetitive elements and 3 untranslated genes. Drld apparently defines a new family of genetic elements.