DHFR and malaria: However, selection in rodent malaria parasites may only be carried out in vivo as they cannot be cultured long term in vitro, reducing the number of positive selectable markers available to three: Toxoplasma DHFR-thymidylate synthase (tgDHFR-TS) and P. berghei DHFR-thymidylate synthase (pbDHFR-TS), which impart resistance to pyrimethamine and, human dihydrofolate reductase (hDHFR) which confers resistance to both pyrimethamine and WR99210.