Intermediate filament (IF) proteins are constituents of the cytoskeleton, conferring resistance to mechanical stress, and are encoded by a dispersed multigene family. In man we have identified two isoforms (180 and 150 kDa) of the IF protein synemin. Synemin α and β have a very short N-terminal domain of 10 amino acids and a long C-terminal domain consisting of 1243 amino acids for the α isoform and 931 amino acids for the β isoform. An intronic sequence of the synemin β isoform is used as a coding sequence for synemin α. Both mRNA isoforms (6.5 and 7.5 kb) result from alternative splicing of the same gene, which has been assigned to human chromosome 15q26.3. Analyses by Northern and Western blot revealed that isoform β is the predominant isoform in striated muscles, whereas both isoforms (α and β) are present in almost equal quantities in smooth muscles. Co-transfection and immunolabeling experiments indicate that both synemin isoforms are incorporated with desmin to form heteropolymeric IFs. Furthermore synemin and desmin are found aggregated together in certain pathological situations.