Steve McCarroll studies how repeated DNA affects biology. Thousands of places in our genomes contain regions in which simple DNA sequences are repeated many times. Such sequences are inherently unstable and can expand in length throughout our lifetimes, changing the properties of the genes in which they reside. McCarroll and his laboratory team seek to understand why this process plays out differently in different types of cells, how it changes the biology of those cells, and how it contributes to brain disorders such as Huntington disease. His team hopes to develop new ways to understand and prevent DNA-repeat disorders.