When patients present with a dementia syndrome at a young age, the experienced clinician will automatically include uncommon dementias in the diagnostic considerations, as familial uncommon dementias due to genetic mutations frequently present as early-onset dementias. This paper highlights why uncommon dementias due to genetic mutations, although marginal in terms of prevalence numbers in the total population, are of significance in the quest to unravel the underlying cause of common dementias such as Alzheimer's disease (AD), dementia with Lewy bodies (DLB), frontotemporal dementias (FTD) and vascular dementia (VaD).