The common incidence-age multistep model of neurodegenerative diseases revisited: wider general age range of incidence corresponds to fewer disease steps.

The extended genealogy tree disclosed three groups of NDs according to the estimated time needed for a step to occur: the stem ND, HD, with 32.5 years/step, the trunk NDs ALS, FTD, PD and CJD, with 6.7-13.7 years/step; and the crown NDs PDM, PDD, AD and DLB, with 2.3-3.8 years/step. Thus, the NDs cluster into three groups according to both the number of steps and the number of years for a step to occur.