The only mew cheat for pokemon yellow is this one: First you have to have a Pokemon that knows Teleport. Then go to Cerulean city, but don't go in the gym just yet. Instead, go up Nugget bridge, and to the right will be a grassy area and a trainer. Go up past the bridge and go to the part where you're facing the trainer, but he can't see you (he'll just be off-screen). Then take one step forward and QUICKLY press start. Use teleport and you will go back to the Cerulean Pokemon Centre. If you try to press the start button, you will realise that it won't work. Go into the gym and fight the first trainer in the water. Go outside and head up Nugget Bridge. Your start screen should pop up as soon as you step on it. When it does, press the B button and you will battle Mew! Mew is only at level 7, but he's worht it! And I'm pretty sure that this glitch won't hurt or erase your game, unlike Missing.No. Trust me, this cheat works. I read it on a forum, and i was a little skeptical myself, but i started my game over again and it really worked! Now my Mew is at level 57! Try it yourself!