BPI and immunoglobulin G4-related sclerosing disease: Our analysis of gene expression in response to steroid treatment showed that the levels of expression of genes encoding bactericidal substances, such as myeloperoxidase, cathepsin G, bactericidal/permeability-increasing protein and lactotransferrin, were lower in IgG4-RD patients prior to steroid treatment than in healthy controls (Table 5).