@moonman That's not entirely correct if by MBR you mean the partition table (as it seems from the context).
BIOS just executes the drive as a programme from the first bytes, then it's up to them what to do.
GRUB on MBR can very well boot from GPT as long as there are additional 2 MiB of space for it (an extra marked partition). Nothing needs to be in the MBR partition table for that.
There's the matter of booting from an MBR partition, but who needs that anyway?