Immunologic assessment of the infant was deemed necessary given that lymphopenia, neutropenia, and anemia have been reported with JAK inhibition.7 Further, in vivo and in vitro studies have demonstrated that tofacitinib can impair the differentiation of CD4+ helper T cells and strongly impair plasmablast development from naïve B cells, which may impact immunoglobulin secretion.8,9 No significant alterations were evident on this baby’s immunological assessment while breastfeeding at 12 weeks of age. The gene discussed is CD4; the disease is lymphopenia.