[vitamin C]
All the great apes share a common frame-shift mutation (I forget whether it's an insertion or a deletion) that makes it impossilbe to synthesize ascorbic acid (vitamin C).
There's a particular enzyme, LGLO, present in all mammals (at least), its gene has been sequenced, and it just happens to be broken in the same place in the great apes (including us).
There are a couple of other mammals that need vit C, guinea pigs and fruit bats, but they have different mutations from us and each other.
OK.