CDKN2A and neoplasm: In two of 10 meningiomas found in adCre;Nf2flox2/flox2;Ink4a*/* mice, features that in human tumors may be called “atypical” (one feature in each tumor) were observed: prominent nucleoli (Figure 1C) and crowding of the cells, but neither had enough atypical features to be called atypical meningioma by the current WHO classification of human tumors.