Nuclease-mediated methods for expanding repeats already present at a genomic locus are provided. Non-human animal genomes, non-human animal cells, and non-human animals comprising a heterologous hexanucleotide repeat expansion sequence inserted at an endogenous C9orf72 locus and methods of making such non-human animal cells and non-human animals through nuclease-mediated repeat expansion are also provided. Methods of using the non-human animal cells or non-human animals to identify therapeutic candidates that may be used to prevent, delay or treat one or more neurodegenerative disorders associated with repeat expansion at the C9orf72 locus are also provided.