Cytosolic 5'-nucleotidase 3 (NT5C3) catalyzes the hydrolysis of pyrimidine nucleoside monophosphates (d)CMP, TMP, and (d)UMP to nucleosides plus orthophosphate. While the enzyme appears to be present in many tissues, it is especially abundant in erythrocytes, where it may function to remove excess pyrimidine nucleotides generated by nucleic acid breakdown, while sparing purine nucleotides needed for red cell energy metabolism. Deficiencies in the enzyme are associated with a form of hemolytic anemia and its inactivation by heavy metals may be responsible for some hematological abnormalities associated with lead poisoning (Marinaki et al. 2001; Rees et al. 2003). The active form of the enzyme is a monomer. It has an absolute requirement for Mg++, and is inactive against purine nucleotides (Amici et al. 1997; Amici and Magni 2002).