The games I have that yellowed are in fact american "Turbo Chips" one is bonks revenge, the other is keith courage, these are the games that I had since 94 or 95, when I lived at home, I had a room facing the west which got alot of sunlight, I would leave my video game stuff hooked up at once, and the turbo at that time I never played often, but I left it at the bottom shelf, with a turbo chip inside, it was this sun exposure that I think made the real half of the turbochip yellow.