What is the composition of yeast extract?

For many decades now many bacteriologists have used yeast extract to successfully culture bacteria. Nowadays, we see multiple examples where yeast extract appears to elicit certain regulation responses and provide with "growth factors" that in absence would prevent cell growth. Despite this, to my knowledge, only a handful of reports describe YE partially but to date, no one seems to take on the task of defining the medium. Thus the question... What is the composition of YE?