Using a new pEnLox vector employed to generate gain-of-function mutants in Arabidopsis thaliana, the AtFAS4 mutant has been obtained and analyzed. The mutant is characterized by super-expression of the At1g33390 gene, which leads to the occurrence of a mutant phenotype – stem fasciation. The level of expression of the AtFAS4 gene in normally developing A.thaliana plants is extremely low thus accounting for almost complete absence of information on EST’s of this gene. The generated AtFAS4 mutant has permitted full-length cDNA of the At1g33390 gene to be obtained and analyzed for the first time.