POU1F1 and hypopituitarism: We show that activation of the ERK/MAPK pathway by expressing Braf p.V600E only in the pituitary gland (Prop1+ve pituitary progenitors cells) or the knock-in allele of the most common human CFC-causing mutation, the hBRAF p.Q257R (CAG:Cre;BrafQ241R/+), results in clear hypopituitarism with a decrease in the cell lineage determination factors Pit1 and Sf1, required for terminal differentiation of somatotrophs (GH+ve), thyrotrophs (TSH+ve), lactotrophs (PRL+ve) and gonadotrophs (LH+ve and FSH+ve).