The functional units of DNA that contain the instructions for making one or more proteins are called genes.

Here's why:

* Genes are segments of DNA: They represent specific sequences of nucleotides within the DNA molecule.

* They contain instructions for protein synthesis: The sequence of nucleotides in a gene determines the sequence of amino acids in a protein.

* They can code for multiple proteins: Some genes have regions that can be spliced together in different ways, leading to the production of multiple protein variations from a single gene.

Let me know if you'd like more details about genes or protein synthesis!