IGHG4 and COVID-19: In module 3, the OXPHOS-related gene (ATP13A3) was found to be positively correlated with genes encoding immunoglobin chains (IGHJ6, IGHG1, and IGHG4) and X-Box Binding Protein 1 (XBP1), which is required for plasmacytic differentiation and crucial for the plasma cell secretory program in patients with severe COVID-19 (Figures 4C, E) (46).