cell population phenotype abnormal phenotype increased mitotic index A cell population phenotype in which the mitotic index is higher than normal. The mitotic index is the proportion of the population undergoing mitosis at any given time. midori 2014-04-23T14:26:27Z FYPO:0003307 Mitotic index is typically determined by microscopy, either by using tubulin immunofluorescence to detect spindles, or by using DAPI staining to detect DNA, where the presence of two nuclei but no septum is interpreted as anaphase.