A protein complex that binds to the Mlu1 cell cycle box (MCB) promoter element, consensus sequence ACGCGN, and is involved in regulation of transcription during the G1/S transition of the cell cycle. In Saccharomyces, the complex contains a heterodimer of the DNA binding protein Mbp1p and the activator Swi4p, and is associated with additional proteins known as Nrm1p, Msa1p, and Msa2p; in Schizosaccharomyces the complex contains Res1p, Res2p, and Cdc10p.