Human p53R2 (hp53R2) is a 351-residue p53-inducible ribonucleotide reductase (RNR) small subunit. It shares >80% sequence identity with hRRM2, the small RNR subunit responsible for normal maintenance of the deoxyribonucleotide (dNTP) pool used for DNA replication, which is active during the S phase in a cell cycle-dependent fashion. But rather than cyclic dNTP synthesis, hp53R2 has been shown to supply dNTPs for DNA repair to cells in G0-G1 in a p53-dependent fashion. The first X-ray crystal structure of hp53R2 is determined to 2.6 A, in which monomers A and B exhibit mono- and binuclear iron occupancy, respectively. The pronounced structural differences at three regions between hp53R2 and hRRM2 highlight the possible regulatory role in iron assimilation and help explain previously observed physical and biochemical differences in the mobility and accessibility of the radical iron center, as well as radical transfer pathways between the two enzymes. The sequence-structure-function correlations that differentiate hp53R2 and hRRM2 are revealed for the first time. Insight gained from this structural work will be used in the identification of biological function, regulation mechanism, and inhibitor selection in RNR small subunits.