In cancer genomics, recurrence of mutations in independent tumor samples is a strong indicator of functional impact. However, rare functional mutations can escape detection by recurrence analysis owing to lack of statistical power. We enhance statistical power by extending the notion of recurrence of mutations from single genes to gene families that share homologous protein domains. Domain mutation analysis also sharpens the functional interpretation of the impact of mutations, as domains more succinctly embody function than entire genes. By mapping mutations in 22 different tumor types to equivalent positions in multiple sequence alignments of domains, we confirm well-known functional mutation hotspots, identify uncharacterized rare variants in one gene that are equivalent to well-characterized mutations in another gene, detect previously unknown mutation hotspots, and provide hypotheses about molecular mechanisms and downstream effects of domain mutations. With the rapid expansion of cancer genomics projects, protein domain hotspot analysis will likely provide many more leads linking mutations in proteins to the cancer phenotype. Display omitted •Comprehensive analysis of cancer mutations in protein domains (http://www.mutationaligner.org)•Identification of new mutation hotspots across homologous domains•Functional coupling of rare, uncharacterized mutations and known oncogenic mutations•Mechanistic clues about downstream effects of mutations in signaling domains Using cancer genomics datasets from thousands of tumor samples in 22 tumor types, Miller et al. analyze somatic missense mutations in protein domains and discover new domain mutation hotspots. By associating mutations in infrequently altered genes with mutations in frequently altered paralogous genes that are known to contribute to cancer, this study provides many new clues to the functional role of rare mutations in cancer.