The intermediate filament protein desmin generates an extra-sarcomeric network in myocytes. Mutations in the desmin gene cause myofibrillar myopathy characterized by desmin-positive aggregates and myofibrillar dissolution. Past analysis revealed that the non-alpha-helical amino-terminal "head" domain of desmin is a vital coordinator of protein assembly. We have now characterized assembly and network-forming properties of five recently discovered myopathy-causing mutations residing in this domain. In vitro analyses with recombinant proteins show that two mutant variants residing in a conserved nonapeptide motif "SSYRRTFGG"-Ser13Phe and Arg16Cys-interfere with assembly by forming filamentous aggregates. Consistent with in vitro data, both mutant proteins are unable to generate a bona fide filament system in cells lacking an intermediate filament cytoskeleton. In cells expressing vimentin or desmin, both mutants firstly fail to integrate into the endogenous filament network and secondly severely affect its cellular localization. The other three mutations-Ser2Iso, Ser46Phe, and Ser46Tyr-influence in vitro filament properties less severely, but in vivo, Ser46Phe and Ser46Tyr impair de novo filament formation. These effects of the "head" mutant proteins on endogenous intermediate filament system and their competition for binding to cellular anchoring structures might explain part of the molecular mechanism that causes disease.