To develop a conditional system for C. parvum based on the E. coli DHFR degradation domain (DDD), we took advantage of the natural resistance of C. parvum to antifolates, CRISPR/Cas9 editing, immunocompromised mouse model of infection, and the inexpensive compound TMP with favorable pharmacological properties (29, 47). The gene discussed is DHFR; the disease is infection.