Genetic studies have revealed many genes linked to both common and rare disease, but to understand how those genes bring about disease and use those insights to help develop therapies, scientists need to know where they are active in the body. Research on single cells can help achieve this goal, by surveying gene activity in specific cell types. Scientists need to profile all cell types and compare them across organs in the body to learn about the full range of human diseases, but this is difficult to do with existing methods.