The RPB6 cDNA of chicken, encoding one of the small subunits common to all three nuclear DNA-dependent RNA polymerases, has been isolated from an expression cDNA library by screening with a differential display derived probe, representing a gene shown to be highly up-regulated in early heart development. The nucleotide sequence of the cDNA isolated predicts a protein sequence of 127 amino acids. This sequence shares 124 amino acids (98% homology) with the human RNA polymerase II subunit 14.4 kDa (RPB6) and hamster hRPB6 and 123 amino acids (97% homology) with Rattus norvegicus RNA polymerase II subunit RPB6. Other conserved motifs in this protein and potential functions of RPB6 are discussed.