The flavin-dependent monooxygenase TetX2 from Bacteroides thetaiotaomicron confers resistance against tetracyclines in aerobically grown Escherichia coli. TetX2 modifies several tetracycline antibiotics by regioselective hydroxylation of the substrates to 11a-hydroxy-tetracyclines. X-ray diffraction data were collected from a native TetX2 crystal and a TetX2 crystal with incorporated selenomethionine to resolutions of 2.5 and 3.0 A, respectively. The native crystal belonged to the triclinic space group P1, with unit-cell parameters a = 68.55, b = 80.88, c = 87.53 A, alpha = 111.09, beta = 98.98, gamma = 93.38 degrees , whereas the selenomethionine-labelled TetX2 crystal belonged to the monoclinic space group P2(1), with unit-cell parameters a = 87.34, b = 68.66, c = 152.48 A, beta = 101.08 degrees .