The flavin-dependent monooxygenase RebC is a key enzyme in the biosynthesis of the indolocarbazole rebeccamycin. The synthesis of rebeccamycin is of great interest as it has been shown to be a natural antitumour agent. The enzyme has been recombinantly expressed in Escherichia coli and purified to homogeneity. Hanging-drop vapour diffusion in combination with microseeding was used to obtain suitable crystals for X-ray diffraction. Data were collected to 2.4 A; the crystals belonged to space group P2(1), with unit-cell parameters a = 63.08, b = 77.85, c = 63.94 A, alpha = gamma = 90, beta = 108.11 degrees .