Today I've learned that there was a compilation containing both Mortal Kombat and Mortal Kombat II for the original Game Boy. What's curious about it is that it was rated T by the ESRB, despite the series' long history of M-rated games, beating Mortal Kombat Vs. DC Universe to the punch by a decade.
I thought, "maybe they didn't think the game was so violent when played through the tiny, black-and-white screen of the low-spec Game Boy". But then there's a port of Mortal Kombat 3 for the handheld, and this time it was actually rated M by the ESRB, so I'm not sure what to think anymore.
OG MK on GB was released pre-ESRB during the window when Nintendo demanded bloodless, sanitized ports for their consoles. It _could_ have been rated "T".
They changed their policy with MK2, which had an M rating on Game Boy. So I don't know what's going on with this collection.