The presence of DHPS mutations in patients without prior TMP-SMX or dapsone use for PCP prophylaxis, the absence of similar mutations in Pneumocystis isolated from other mammalian species, and the impact of geography on DHPS genotype have substantial implications for disease transmission (i.e., person-to-person transmission) that are beyond the scope of this review (32–35). This evidence concerns the gene DHPS and pneumocystosis.