A novel human protein factor CIB (CCG1-interacting factor B) was isolated using the yeast two-hybrid system. CIB was purified and subsequently crystallized using the vapor-diffusion method. Dynamic light-scattering studies of CIB revealed this protein to be monodispersive and present as a monomer in solution. Thin plate-like crystals were grown by the hanging-drop method in the presence of ammonium sulfate as a prime precipitant. The crystals belong to the space group P2(1)2(1)2(1), with unit cell dimensions of a = 43.88 Angstrom, b = 44.69 Angstrom, and c = 111.50 Angstrom and contain one molecule in the asymmetric unit.