Much of stem cell research over the past decade has focused on Mendelian disorders — those caused by a single gene, such as cystic fibrosis, muscular dystrophy, and Huntington’s disease. But as genome-wide association studies continue to reveal, most conditions are more complex, arising from dozens or hundreds of genetic mutations working together to cause disease. To understand how someone can inherit these disorders, it’s necessary to study how these mutations contribute on a cellular level.