The IMTCGS is a two-tiered system in which low-grade MTCs are defined as tumors with an MI of < 5 mitoses/2 mm2, a Ki67 proliferative index of < 5%, and absence of tumor necrosis, whereas high-grade MTCs are those with at least one of the following three features: MI ≥ 5 mitoses per 2 mm2, Ki67 proliferative index ≥ 5%, or tumor necrosis (Data Supplement). The gene discussed is MKI67; the disease is neoplasm.