APOE and obesity due to melanocortin 4 receptor deficiency: PSRC1 deficiency significantly increased the ratio of Firmicutes to Bacteroidetes (0.45 versus 0.24 in apoE−/− mice, p < .01) (Figure 2c), which is considered as a hallmark of obesity and atherosclerosis.23 Overall, the difference in the microbiota consisted of 589 common species, 140 endemic species in apoE−/− mice and 256 in DKO mice (Figure 2d).