The eukaryotic RPB3 subunit of RNA polymerase (RNAP) II is involved in the assembly of RNAP subunits. RNAP is a large multi-subunit complex responsible for the synthesis of RNA. It is the principal enzyme of the transcription process, and is a final target in many regulatory pathways that control gene expression in all living cells. At least three distinct RNAP complexes are found in eukaryotic nuclei: RNAP I, RNAP II, and RNAP III. RNAP II is responsible for the synthesis of mRNA precursor. The RPB3 subunit is similar to the bacterial RNAP alpha subunit in that it contains two subdomains: one subdomain is similar the eukaryotic Rpb11/AC19/archaeal L subunit which is involved in dimerization, and the other is an inserted beta sheet subdomain. The RPB3 subunit heterodimerizes with the RPB11 subunit, and together with RPB10 and RPB12, anchors the two largest subunits, RPB1 and RPB2, and stabilizes their association.