BTK and isolated agammaglobulinemia: Other PH domains with known links to disease include the PDPK1 (3-phosphoinositide–dependent protein kinase 1), P-Rex1 [phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 protein], and IQSEC1 (IQ motif and SEC7 domain–containing protein 1) PH domains in cancer and intellectual disability; the BTK (Bruton’s tyrosine kinase) PH domain in autoimmune disease and X-linked agammaglobulinemia; and the FGD1 (FYVE, RhoGEF, and PH domain–containing protein 1) PH domain in faciogenital dysplasia (12, 14–20).