MKI67 and pituitary gland adenoma: The 4th edition of WHO classification of pituitary tumors has removed the term atypical pituitary adenoma (APA) for the difficulty and inconsistency in determining proper cutoff of the diagnostic criteria being used before (4, 6–9), in the previous 3rd version, APA is defined as tumors that display invasive growth, Ki-67 index >3%, extensive nuclear staining for p53 and elevated mitotic activity (10), which is vague.