Final answer:
PrPC is encoded by a cellular gene, rich with alpha-helices, and converted by PrP^Sc into a disease-causing form.
Step-by-step explanation:
The true statement about PrPC is that it is encoded by a cellular gene, it is rich with alpha-helices, and it is converted by PrP^Sc into a disease-causing form. Therefore, the correct option is All of the above.