gas-1(fc21) or nduf-7(et19) P0 animals were randomly mutagenized with ethyl methanesulfonate (EMS) at 21% oxygen. F2 progeny were transferred to the non-permissive 50% oxygen tension and selected for growth. NDUFS4/lpd-5 null C. elegans are not fertile at any oxygen tension, making a forward genetic screen challenging.