Dephosphorylates specifically the 5' and 2'(3')-phosphates of uracil and thymine deoxyribonucleotides, and so protects mitochondrial DNA replication from excess dTTP. Has only marginal activity towards dIMP and dGMP. Homodimer. Highly expressed in heart, brain and skeletal muscle. Detected at very low levels in kidney and pancreas. Belongs to the 5'(3')-deoxyribonucleotidase family.