Prompted by the knowledge that murine pDC development is dependent upon Ikzf1, here we investigate whether IKZF1 mutation or inhibition with lenalidomide causes pDC deficiency in humans, using phenotypic and functional analyses performed on patients with IKZF1 haploinsufficiency, those receiving lenalidomide, or on progenitor cell cultures exposed to lenalidomide in vitro. The gene discussed is PDC; the disease is hyperinsulinemic hypoglycemia, familial, 4.