Mycobacterium tuberculosis open reading frame Rv1264 encodes an adenylate cyclase that exhibits its highest enzymatic activity at an acidic pH of 6.0. This is the pH M. tuberculosis encounters in the phagosome. Consequently Rv1264 has been suggested to sense the phagosomal milieu resulting in adaption of M. tuberculosis to its intracellular niche. A targeted knock-out mutant deficient in Rv1264, however, exhibits wild-type virulence.