SDC1 and systemic lupus erythematosus: Furthermore, the histological examination of lymph node tissues in KLH-immunized mice (Fig. 6i) and lupus mice (Supplementary Fig. 7p) also indicated that IRF4 was predominantly expressed in CD138+ PCs, an expression pattern that was apparently diminished in the absence of Ythdf1. In line with these observations in mice, IRF4 was also principally expressed in human PCs, the level of which was much higher in SLE peripheral PCs than in HCs (Fig. 6j).