C57BL/6 mice with genetic disruption of the PD-1 gene are more susceptible to Mtb infection with both severe pathology and greater Mtb burden (213, 214, 216). The PD-L1 gene knockout mice do not have as severe TB than the PD-1 knockout mice, perhaps because PD-L2 gene is still intact (Figure 6) (213, 281). See above for more detailed discussion. This evidence concerns the gene PDCD1 and tuberculosis.