1. In a large ziploc bag, combine soy sauce, sugar and ketchup. Add tenderloin. Seal bag and turn to coat. Marinate at least over night, preferrably longer though.
2. Preheat oven to 425 (I used the grill instead).
3. Place pork in 13 x 9 greased baking dish. (I put the remaining marinade in a saucepan, brought to a boil and added some cornstarch to thicken it.) I served the sauce on top of the pork.
4. Bake, uncovered, at 425 for 35-40 minutes OR until meat thermometer reads 160. Let rest for 5 minutes before slicing.