Precursor proteins are inactive forms of proteins that require specific modifications to become functional. These proteins often undergo processes such as cleavage or folding, which transform them into their active states. This transformation is crucial for various biological functions, including hormone regulation and enzyme activity. In many cases, precursor proteins are synthesized in the endoplasmic reticulum and then transported to other cellular locations for activation. For example, the proinsulin precursor is converted into active insulin through enzymatic cleavage. Understanding precursor proteins is essential for studying cellular processes and developing medical treatments.