ERBB4 and myeloid sarcoma: We first compared the ErbB4 expression in PBMCs of patients with RR-MS and age-matched HC by flow cytometry and found that the mean fluorescence intensity (MFI) of ErbB4 in PBMCs of patients with RR-MS was significantly lower (37.8 ± 2.8) than in HC (60.0 ± 5.1, P = 0.002) (Figure 1(a)).