Frey's theorem states that every Abelian category is a subcategory of some category
of modules over a ring. Mitchell
(1964) has strengthened this, saying every Abelian category is a full subcategory
of a category of modules over a ring.
This entry contributed by John Renze
More things to try:
Renze, John. "Freyd's Theorem." From MathWorld--A Wolfram Web Resource, created by Eric
W. Weisstein. https://mathworld.wolfram.com/FreydsTheorem.html