The smallest segment of DNA that codes for a protein is called a gene.

Here's why:

* Genes contain the instructions: A gene is a specific sequence of DNA nucleotides that provides the blueprint for building a particular protein.

* Proteins are essential: Proteins perform a wide range of functions in the body, from building structures to catalyzing chemical reactions.

* Other DNA sequences: While genes are the core coding units, other DNA sequences (like regulatory elements) are involved in controlling when and how genes are expressed.

Let me know if you have any other questions!