CYP27B1 and systemic lupus erythematosus: 25(OH)D is the main form of vitamin D in blood serum, and CYP27B1 can hydroxylate 25(OH)D to its active form: 1,25(OH)2D. The serum level of 25(OH)D was significantly lower in SLE patients than in HCs (p = 0.003) as shown in Figure 3.