Benign adult familial myoclonic epilepsy type 1 (BAFME1) is an autosomal dominant, adult-onset neurological disease caused by SAMD12repeat expansion. In BAFME1, anticipation, such as the earlier onset of tremor and/or seizures in the next generation, was reported. This could be explained by intergenerational repeat instability, leading to larger expansions in successive generations. We report a four-generation BAFME1-affected family with anticipation. Using Nanopore long-read sequencing, detailed information regarding the sizes, configurations, and compositions of the expanded SAMD12repeats across generations was obtained. Unexpectedly, a grandmother-mother-daughter triad showed similar repeat structures but with slight repeat expansions, despite quite variable age of onset of seizures (range: 52–14 years old), implying a complex relationship between the SAMD12repeat expansion sequence and anticipation. This study suggests that different factor(s) from repeat expansion could modify the anticipation in BAFME1.