CYS4 affects drug-sensitivity and glutathione levels. (a) Relative resistance to atenolol for the parental strains (M, Y, S); for the deletion of CYS4 in M (MΔ), Y (YΔ), and S (SΔ); and for the M allele replacement (M+). (b) Total glutathione levels for the parental strains (M, Y, S), for the deletion of CYS4 in M (MΔ), and for the M allele replacement (M+).