K430R-Btk restores serum Ig levels in Btk/SLP-65 double-mutant mice. Serum concentrations of IgM, IgG1, IgG2a, IgG2b, IgG3, and IgA, as determined by ELISA, are shown for the indicated mouse strains. Mice were 2 months of age and each symbol represents an individual animal. □ indicates Btk⁺; ▵, Btk^-; ○, K430R-Btk; ▪, Btk⁺ SLP-65^-; ▴, Btk^- SLP-65^-; and •, K430R-Btk SLP-65^-.