The hbhA gene of Mycobacterium tuberculosis is specifically upregulated in the lungs but not in the spleens of aerogenically infected mice. Infect Immun. 2006 May;74(5):3006-11

We report that hbhA is differentially regulated during Mycobacterium tuberculosis infection. Upregulation was observed in epithelial cell infection but not in macrophage infection and in the lungs but not in the spleens of infected mice, and it was greater during the early steps of infection, when bacilli disseminate from the site of primary infection.