MKI67 and pituitary tumor: Methodological problems, interpretational difficulties, and use of different cut-off values in different studies [73, 74] may explain why the interpretation of Ki-67 has recently been reformulated, moving from the cut-off of 3% in the previous WHO classification of pituitary tumors [15] to the estimation of the percentage of Ki-67 positive nuclei without precise cut-off in the 2017 WHO classification [2].