Daniel Korenfeld, Kate Roussak, Sabrina Dinkel, Tiphanie P. Vogel, Henry Pollack, Joseph Levy, Jennifer W. Leiding, Joshua Milner, Megan Cooper and Eynav Klechevsky J Immunol October 15, 2021, ji2000841; DOI: https://doi.org/10.4049/jimmunol.2000841 Key Points STAT3 gain-of-function causes a novel disorder of dendritic cells and monocytes. STAT3 gain-of-function patients lack nonclassical monocytes and certain dendritic cells. STAT3 mutations cause altered dendritic cell differentiation.