XBP1 and myeloid sarcoma: There were no differences between the proportions of these B cell subsets when comparing cells from patients with MS and healthy controls (21, 36) (Supplemental Figure 2C), and, similarly, no differences were observed in the expression levels of the transcription factors IRF4, PRDM1, and XBP1 with regard to the plasmablast developmental program (37, 38) (Supplemental Figure 2D).