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Framework Region
In molecular biology, framework regions are a subdivision of the fragment antigen binding region (Fab) of the antibody. Located on the tips of the Y-shaped molecule, the framework regions are responsible for supporting the binding of the hypervariable region (CDR) of the Fab to the antigen. The side chains of certain amino acids in the framework help facilitate the stable binding of the antibody to the antigen and define the position of the CDR so they are exposed on the surface of the chain ready to bind to an antigen.

The framework region is made up of 4 amino acid segments between the 3 CDR regions of the Fab. The framework region makes up about 85% of the variable region. To increase its stability, the framework region has less variability in its amino acid sequences compared to the CDR. There are 4 framework regions found on each antibody located on the two variable heavy domains and the two variable light domain.