Protein domains are functional and structural units within a protein that can act independently or combine with other domains to form multi-domain proteins. These domains are typically responsible for specific functions within the protein, such as binding to other molecules, enzymatic activity, or signaling. Studying protein domains is important for understanding the structure and function of proteins, as well as for predicting the effects of mutations or changes in protein sequence. Research in this area includes identifying and characterizing new protein domains, predicting domain structures computationally, and studying the evolutionary relationships between different domains. Protein domain research plays a crucial role in fields such as bioinformatics, structural biology, and drug discovery.