Ribonucleotide Reductase, R2/beta subunit (RNRR2) is a member of a broad superfamily of ferritin-like diiron-carboxylate proteins. The RNR protein catalyzes the conversion of ribonucleotides to deoxyribonucleotides and is found in all eukaryotes, many prokaryotes, several viruses, and few archaea. The catalytically active form of RNR is a proposed alpha2-beta2 tetramer. The homodimeric alpha subunit (R1) contains the active site and redox active cysteines as well as the allosteric binding sites. The beta subunit (R2) contains a diiron cluster that, in its reduced state, reacts with dioxygen to form a stable tyrosyl radical and a diiron(III) cluster. This essential tyrosyl radical is proposed to generate a thiyl radical, located on a cysteine residue in the R1 active site that initiates ribonucleotide reduction. The beta subunit is composed of 10-13 helices, the 8 longest helices form an alpha-helical bundle; some have 2 addition beta strands. Yeast is unique in that it assembles both homodimers and heterodimers of RNRR2. The yeast heterodimer, Y2Y4, contains R2 (Y2) and a R2 homolog (Y4) that lacks the diiron center and is proposed to only assist in cofactor assembly, and perhaps stabilize R1 (Y1) in its active conformation.