The Joker (DC Animated Universe) encoded his DNA into a microchip that he implanted into Tim Drake's spinal cord, allowing the Clown Prince of Crimes to slowly but surely "reborn" in Drake's body.
When Osmosians (Ben 10) attempt to absorb too many life forms at once, their own DNA will be overwritten with alien DNA, causing them to become a chimeric mixture of those creatures.
Kevin 11,00 (Ben 10) in his Mutation form.
Devlin Levin (Ben 10) in his Mutation form.
Using the Omnitrix, Ben Tennyson (Ben 10) can temporarily overwrite his DNA with that of aliens, turning him into said aliens.
Matthew Kim (The Flash) can transfer pieces of DNA into other beings with a simple touch...
...unintentionally overwriting Iris West's DNA with Barry Allen's, giving her Barry's speed.
In addition to curing her of X Parasite infestation, the Metroid vaccine (Metroid Fusion) also partially overwrote Samus' DNA with that of a Metroid, resulting in her gaining Metroid traits, such as a vulnerability to extreme cold.
Dr. Eggman's Evil Cookies (Sonic Boom), when infused with DNA from a specific person, overwrite the DNA of those who consume them, turning them into a clone of said person...
...with Sonic's friends being transformed into clones of Eggman after eating cookies infused with Eggman's DNA...
...and Eggman himself being transformed into a clone of Sonic after being force-fed a cookie infused with Sonic's DNA.