On the other hand, one can also observe that several animal-adapted strains, such as M. mungi, M. suricattae, and the dassie bacillus from clade A1 and M. microti from clade A2 show variably sized deletions in the core ESX-1 locus (Brites et al. 2018, Orgeur et al. 2021), suggesting that natural infection cycles in certain animal hosts might not need the prominent ESX-1 virulence functions that are essential for M. tuberculosis infection in humans. This evidence concerns the gene ESX1 and infection.