To determine whether the ability of the MX2T151A mutant to restrict HIV-1 infection in the absence of CypA-CA binding is due to the lack of a phosphate group at this residue or due to the inability to hydrolyze GTP, we assessed the antiviral activity of the phosphomimetic T151D MX2 mutant in HT1080 and HeLa cells (Figs 4, S10, and S11). The gene discussed is MX2; the disease is HIV-1 infection.