The Rv1625c gene product is an adenylyl cyclase identified in the genome of Mycobacterium tuberculosis strain H37Rv. It shows sequence similarity to the mammalian nucleotide cyclases and functions as a homodimer, with two substrate-binding sites at the dimer interface. A mutant form of the catalytic domain of this enzyme, K296E/F363R/D365C (KFD-->ERC), was overexpressed in Escherichia coli cells in a soluble form. Crystals were obtained using the hanging-drop vapour-diffusion method with PEG 8000 as a precipitant. The protein crystallized in space group P4(1), with unit-cell parameters a = b = 71.25, c = 44.51 A. X-ray diffraction data were collected to a resolution of 3.4 A and the structure has been solved by the molecular-replacement method using a previously built theoretical model of the protein as the search molecule.