Myosin is plentiful in muscle cells, but is not found at all in lens cells. Lens cells contain large amounts of crystallins, which are not found in muscle cells. Why do some proteins appear in one type of cell but not in another?

The answer is: During differentiation, certain genes are activated to produce proteins that enable specific functions.

A mutation in a plant that could cause negative outcomes could be the plant has less leaves, or isn't hardy enough, like other plants. It would die. A plant from an area like Chernobyl, that has been affected by radiation could poison other animals, or pass that gene along, and ultimately kill that strand of plant.