Here's a breakdown:

* Gene: A unit of heredity that carries the instructions for building a protein.

* Exon: A coding sequence within a gene that will be transcribed into mRNA and translated into a protein.

* Intron: A non-coding sequence within a gene that is transcribed into mRNA but then removed (spliced out) before translation.

So, while the entire gene contains the instructions for a protein, only the exons actually hold the code that will be used to build the protein.