In molecular biology, a domain is a distinct, functional, and independently folding unit within a larger protein or nucleic acid molecule. Domains are typically composed of 50-200 amino acids in length and are often responsible for carrying out specific functions within a larger protein.

Proteins can be composed of one or more domains, each of which may have a unique structure and function. For example, the Notch protein contains multiple domains, including the extracellular domain, transmembrane domain, and intracellular domain, each of which performs specific functions in the Notch signaling pathway.

Domains can also be found in nucleic acid molecules, such as DNA and RNA, where they are often involved in binding to other molecules or regulating gene expression.

Understanding the domains within a protein or nucleic acid molecule is important for understanding its overall structure and function. By identifying and characterizing the individual domains within a larger molecule, researchers can gain insights into how the molecule works and potentially identify targets for therapeutic intervention.