The pivotal role of the Th1 response is best exemplified by the damaging clinical course of tuberculosis (TB) in the HIV epidemic, where low CD4 T cell counts predict progression from latent tuberculosis infection (LTBI) to active TB (Ellis et al., 2017), and also in M. leprae infection, where Th1/Th2 polarisation is crucial for the two clinical variants –paucibacillary tuberculoid leprosy and the multibacillary lepromatous leprosy (Pinheiro et al., 2018). The gene discussed is CD4; the disease is tuberculoid leprosy.