However, Ki67 has been introduced in the last 2021 WHO classification (5th Edition) as a parameter helping the distinction among different entities, with a range of percentages of Ki67 positive cells indicated for each entity: up to 5% for typical carcinoid, up to 30% for atypical carcinoid, and > 30% for NECs (either small or large cell subtypes) [70]. This evidence concerns the gene MKI67 and carcinoid tumor.