HESX1 is a paired-like homeodomain transcription factor that functions as a repressor of PROP1-mediated gene stimulation. Mutations in HESX1 have been implicated in cases of septooptic dysplasia and congenital hypopituitarism. All mutations in HESX1 identified to date have resulted in impaired DNA binding and defective HESX1 action. We have identified a novel HESX1 mutation in genomic nucleotide position 1684 (g.1684delG), which results in a mutant protein with increased DNA binding. In turn, this mutation causes increased repression of PROP1-dependent gene activity. These data suggest that enhancement of transcriptional repression during pituitary organogenesis is a novel mechanism for the development of congenital pituitary disorders.