PRDM1 and immunoglobulin G4-related sclerosing disease: In patients with IgG4-RD, B-cell lymphoma 6 protein (BCL6) and PRDM1, a pair of mutually restrictive transcription factors involved in B-cell differentiation, showed opposite expression in CD19+CD24−CD38hi plasmablasts/plasma cells compared with the other three B-cell subsets, with BCL6 expression being lower and PRDM1 expression higher in CD19+CD24+CD38hi plasmablasts/plasma cells.