The most common mutation in WHIM syndrome is a truncation at Arg334 (R334X) (101) which, similar to the CXCR4S338X truncation, promotes increased CXCL12-mediated signaling, G protein interactions and ERK and AKT activation, and decreased GRK6 associations, β-arrestin2 interactions and impaired receptor internalization (98–105). The gene discussed is CXCL12; the disease is WHIM syndrome.