The cloning of the human RNA polymerase I 40 kDa subunit, and the comparison of its amino acid sequence to other related RNA polymerase subunits are described. The amino acid sequence of hRPA40 has high homology to the mouse RNA polymerase I 40 kDa subunit (93%), to two Arabidopsis thaliana subunits (47%), the yeast RPC40 subunit (46%) and the human RNA polymerase II hRPB33 subunit (40%). Southern blot analysis shows that this gene is single copy and Northern blot analysis indicates that the mRNA of 1.3 kb is expressed in different cell types.