In biology, a Src homology domain is one of the two small protein binding domains found in the Src oncoprotein. Homologs of both the Src homology 2 and Src homology 3 domains are found in numerous other proteins. The Src homology 1 domain was an early name of the protein kinase domain. In biology, a Src homology domain is one of the two small protein binding domains found in the Src oncoprotein. Homologs of both the Src homology 2 and Src homology 3 domains are found in numerous other proteins. The Src homology 1 domain was an early name of the protein kinase domain.