Members of the Myc oncoprotein network (c-Myc, Max, and Mad) play important roles in proliferation, differentiation, and apoptosis. We expressed chimeric green fluorescent protein (GFP) fusions of c-Myc, Max, and three Mad proteins in fibroblasts. Individually, c-Myc and Mad proteins localized in subnuclear speckles, whereas Max assumed a homogeneous nuclear pattern. These distributions were co-dominant and dynamic, however, as each protein assumed the pattern of its heterodimeric partner when the latter was co-expressed at a higher level. Deletion mapping of two Mad members, Mad1 and Mxi1, demonstrated that the domains responsible for nuclear localization and speckling are separable. A non-speckling Mxi1 mutant was also less effective as a transcriptional repressor than wild-type Mxi1. c-Myc nuclear speckles were distinct from SC-35 domains involved in mRNA processing. However, in the presence of co-expressed Max, c-Myc, but not Mad, co-localized to a subset of SC-35 loci. These results show that Myc network proteins comprise dynamic subnuclear structures and behave co-dominantly when co-expressed with their normal heterodimerization partners. In addition, c-Myc-Max heterodimers, but not Max-Mad heterodimers, localize to foci actively engaged in pre-mRNA transcription/processing. These findings suggest novel means by which Myc network members promote transcriptional activation or repression.