Neurofilaments take highly ordered structures composed of parallel mays of 10 nm filaments linked to each other with frequent cross-bridge. It is composed of three components named NF-L, NF-M and NF-H. NF-L is able to form filamentous network alone in Sf9 cells, while M could not. To identify which domain is essential for the assembly of NF-L, two chimera proteins named ML and MML were constructed: ML was composed of the head domain of NF-M and other domains of NF-L; MML was composed of the head and Coil-1 domains of NF-M and Coil-2 and tail domains of NF-L. ML was not only able to form filaments in Sf9 cells, but also co-assemble with NF-M into parallel filamentous bundles. MML could not assemble into filaments. Thus the Coil-1 domain of NF-L was essential for its assembly.