The pituitary-specific transcription factor Pit-1 (pituitary-specific factor 1) is known to play a key role in the differentiation of PRL-, GH-, and TSH-secreting cells, and in the regulation of expression of the corresponding genes. In recent years, 12 distinct mutations of the Pit-1 gene have been shown to be responsible for a phenotype of multiple congenital pituitary hormone deficiency involving PRL, GH, and TSH. We had previously identified, in four siblings with GH, PRL, and TSH deficiencies, a mutation (F135C) resulting in a single amino acid change within the POU-specific binding domain of the Pit-1 molecule. In the present report, we have explored the functional effect of the F135C mutation. In vitro activity tests performed by transfection in human HeLa cells showed decreased transactivation capacity on the PRL, GH, and Pit-1 genes. The DNA binding experiments performed by gel shift showed that the F135C mutation generated a protein capable of binding to DNA response elements. To analyze how the F135C mutation might affect functionality of the transcription factor despite a normal DNA binding, we used a structure modelization approach and also analyzed two other Pit-1 mutant proteins (F135A and F135Y). The loss of functionality in these two mutants was similar to that of F135C. This finding was in keeping with our molecular modeling studies. According to structural data derived from the crystallographic analysis of the DNA/Pit-1 POU domain complex, the conformation of the first helix of the F135C-mutated POU-specific domain could be perturbed to such an extent that any interaction with other transcription cofactors might be definitively prevented.