STAT1 and progressive multifocal leukoencephalopathy: Rituximab also has a black box warning for progressive multifocal leukoencephalopathy (PML), and this risk should be considered prior to use in patients with other risks such as IEI with increased risk of PML (e.g., STAT1 gain-of-function [GOF], DOCK8 deficiency, and severe CD4 lymphopenia) (29).