Abstract: Alzheimer disease (AD) is a devastating neurodegenerative disorder with a complex genetic underpinning. While most cases are late-onset and associated with aging, early-onset AD presents a unique and often more aggressive form of the disease. A specific genetic variant, the APOE ɛ4 allele, has been established as a major risk factor for AD. This comprehensive analysis explores the influence of rare genetic variants, particularly APOE ɛ4 alleles, on the emergence and clinical manifestations of early-onset AD. Understanding the genetic factors contributing to early-onset AD is critical, as it affects individuals in their prime years, imposing substantial burdens on both patients and their families. The age of onset is a critical determinant of AD clinical presentation. In typical late-onset AD, episodic memory impairment is the primary symptom. However, a subset of early-onset AD patients exhibits non-memory symptoms, including language difficulties, visuospatial impairments, or executive function deficits.