Well-described mutations in MHC class II (bare lymphocyte syndrome), recombination activating genes (RAG) −1 and −2, signal transducer and activator of transcription (STAT) −3, and IL-21R that can be modeled in genetically engineered mice and P. murina infection, provide evidence of genetic susceptibility to Pneumocystis infection [55]. Here, IL21R is linked to Pneumocystis infectious disease.