Nearly all attempts to extend the present framework of physics predict the existence of new interactions which are composition dependent and therefore violate the Equivalence Principle.
Equivalence Principle  tests are by far the most sensitive low energy probes of such new physics, which is consistent with the fact that General Relativity is based on the Equivalence Principle, assumed by Einstein (1907) as the
“HYPOTHESIS OF COMPLETE PHYSICAL EQUIVALENCE between a gravitational field and an accelerated reference frame”
Scientific relevance of the Equivalence Principle (IV)