Administration of SLF80821178 tomice induced ∼50% reduction in circulating lymphocyte counts,recapitulating the lymphopenia characteristic of Spns2 null animals.Molecular modeling studies suggest that SLF80821178 bindsSpns2 in its occluded inward-facing state and forms hydrogen bondswith Asn112 and Ser211 and π stacking with Phe234. The gene discussed is SPNS2; the disease is lymphopenia.