Type-VI intermediate filament (IF) which plays an important cytoskeletal role within the muscle cell cytoskeleton. It forms heteromeric IFs with desmin and/or vimentin, and via its interaction with cytoskeletal proteins alpha-dystrobrevin, dystrophin, talin-1, utrophin and vinculin, is able to link these heteromeric IFs to adherens-type junctions, such as to the costameres, neuromuscular junctions, and myotendinous junctions within striated muscle cells. Interacts with GFAP and VIM (By similarity). Isoform 1 interacts with TLN1 and VCL. Isoform 2 interacts with DES and DTNA. Isoform 1 and isoform 2 interact with DMD and UTRN. There are at least two distinct SYNM subpopulations, one in which SYMN interacts with DES within the Z-lines, and another in which it interacts with both DTNA and DES at the costamere. Isoform 2 is strongly detected in adult heart, fetal skeletal muscles and fetal heart. Isoform 1 is weakly detected in fetal heart and also in fetal skeletal muscle. Isoform 1 and isoform 2 are detected in adult bladder (at protein level). The mRNA is predominantly expressed in heart and muscle with some expression in brain which may be due to tissue-specific isoforms. In lens, first detected at 16 weeks when expression is weakly and uniformly distributed. Subsequently, expression becomes much stronger in the epithelium of the anterior part at 25 weeks and later. In retina, weakly expressed at 15 weeks in the nerve fiber and ganglion cell layers (NFL and GCL). From 25 weeks onwards, much stronger expression is observed in the endfeet of Mueller cells, the NFL, and GCL, and much lower expression is observed in a minor subpopulation of cells in the inner cell layer (INL). At 30 and 36 weeks, expression remains in the neural retina, and subsequently becomes stronger in the NFL, GCL, and INL and is decreased in Mueller cells. At 36 weeks, also expressed at the external border of the outer nuclear layer (ONL) (at protein level). Belongs to the intermediate filament family.