it depends solely on what model GENESIS you have.
i have a U.S. first-edition, got it when they first hit TOYS-R-US
in christmas of '89.
it'll boot any JPN title without a hitch.
if this is any help in tracking down one of your own,
mine's a MODEL 1601.
oh, and yes, i had to widen the cart slot a tad to accept
the JPN carts' rounded corners, as opposed to the U.S. carts' angled ones.
five minutes with an X-ACTO, couldn't be easier.
*unscrew the top case half, disconnect the lead to the POWER LED (it just slides on/off the LED prongs)
*unscrew and remove the cartridge guide cage from the top body half (it also holds the spring-loaded cart-slot doors)
*start whittling away at those angled corners on the body half.
*slap it all back together, and you should be in business!