Here's why:

* All human cells (except for red blood cells) contain DNA. DNA is the molecule that holds our genetic information, and genes are segments of this DNA.

* Cheek cells are human cells. They are a type of epithelial cell that lines the inside of your mouth.

* Therefore, cheek cells contain DNA and thus genes.

This is why cheek swabs are often used for DNA testing. The cells collected contain enough genetic material for analysis.