The protein universe as we know is composed of folded structures and intrinsic disordered regions. The latter may adopt structures upon interaction with binding partners. In addition, some proteins contain C‐terminal extensions which act as independent functional units in the context of the protein. Since their activity does not depend on the protein context they can be considered as peptides in proteins. To illustrate this principle, we here discuss the C‐terminal extensions of IgM antibodies which dictate their assembly and the molecular chaperones Hsp90, Hsp70, and Hsp104 which use C‐terminal peptide extensions as a docking site for interaction with different co‐chaperones.