['Genetic abnormalities of the pituitary specific transcription factor, Pit-1, have been reported in several patients with GH, prolactin (PRL) and TSH deficiencies. The most common is a mutation altering an arginine to a tryptophan in codon 271 (R271W) in one allele of the Pit-1 gene. According to the previous in vitro expression study, R271W acted as a dominant negative inhibitor of the wild type to activate the GH promoter. However, healthy carriers with this mutation, who should be affected by the dominant negative effect of R271W, have also been reported. The aim of this study was to clarify in more detail the function of this mutant form of Pit-1.', "Transcriptional activity of R271W for the expression of Pit-1-associated genes was investigated in COS7 cells with the aid of transient transfection assays. The 1.8 kb rat GH, 0.6 kb rat PRL or 1.9 kb rat PRL 5'-flanking regions were inserted upstream of the luciferase reporter gene and were used for functional analysis of R271W. Another reporter gene containing seven Pit-1 responsive elements was also used. The same experiments were also performed using JEG3 and CHO cells.", 'We could not confirm the dominant negative effect of R271W on wild type Pit-1. Furthermore, our expression study revealed that R271W could activate the promoters of GH and PRL genes to levels similar to the wild type.'] Does the R271W mutant form of Pit-1 act as a dominant inhibitor of Pit-1 action to activate the promoters of GH and prolactin genes?