Genesis eprom carts

...and if they are, does anyone know how to make one?

Sorry if this is a common question, but I see people talking about SMS ones all the time.
Of corse how do you think they make a cartridge in the frist place ever taken a cartridge apart , all it is an eprom .
Most mass-produced cartridges are made with mask ROM, not EPROM (which eventually loses its contents due to charge leakage). Prototypes and other short runs are often done with EPROM though.