The basic-helix-loop-helix-leucine zipper (bHLHZip) proteins Myc, Mad and Mnt are part of a transcription activation/repression system involved in the regulation of cell proliferation. The function of these proteins as transcription factors is mediated by heterodimerization with the small bHLHZip protein Max, which is required for their specific DNA binding to E-box sequences. We have identified a novel Max-interacting protein, Mga, which contains a Myc-like bHLHZip motif, but otherwise shows no relationship with Myc or other Max-interacting proteins. Like Myc, Mad and Mnt proteins, Mga requires heterodimerization with Max for binding to the preferred Myc-Max-binding site CACGTG. In addition to the bHLHZip domain, Mga contains a second DNA-binding domain: the T-box or T-domain. The T-domain is a highly conserved DNA-binding motif originally defined in Brachyury and characteristic of the Tbx family of transcription factors. Mga binds the preferred Brachyury-binding sequence and represses transcription of reporter genes containing promoter-proximal Brachyury-binding sites. Surprisingly, Mga is converted to a transcription activator of both Myc-Max and Brachyury site-containing reporters in a Max-dependent manner. Our results suggest that Mga functions as a dual-specificity transcription factor that regulates the expression of both Max-network and T-box family target genes.