The present invention relates to a mutant of a parent filamentous fungal cell, comprising a coding sequence of a polypeptide of interest under the transcriptional control of a promoter regulated by a transcription factor, wherein the gene encoding the transcription factor is modified in the parent filamentous fungal cell to produce the mutant rendering the mutant deficient in the production of the transcription factor, which increases the productivity of the mutant in the production of the polypeptide of interest, and/or reduces or eliminates the cellulase-negative phenotype in the resulting mutant. The present invention also relates to a method for constructing such a mutant and a method of producing a polypeptide of interest with such a mutant.