Mutational signatures have been identified in cancer genomes, providing information about the causes of cancer and treatment vulnerabilities. This protocol describes an assay to determine the genotoxic mechanisms underlying these signatures using cord-blood derived hematopoietic stem and progenitor cells (CB-HSPCs). CB-HSPCs have a low mutation background, enabling sensitive detection of mutations. First, CB-HSPCs are exposed in vitro, sorted, and clonally expanded. This expansion enables whole-genome sequencing to detect the mutation load and respective patterns induced during genotoxic exposure. For complete details on the use and execution of this protocol, please refer to de Kanter et al. (2021).