DHFR and infection: Despite helping to distinguish colonization from infection, qPCR assays show variable performance [8], which depends on the type of PJ gene—multi-copy mitochondrial (i.e., mitochondrial small subunit (mtSSU) of ribosomal RNA) versus single-copy nuclear (e.g., beta-tubulin or dihydrofolate reductase (DHFR)) encoding gene—targeted by the assay.