  We provide a framework for studying the interplay between concordance and
positive mutation and identify some of the basic structures relating the two.
  The fundamental result in understanding knot concordance is the structure
theorem proved by Levine: for n>1 there is an isomorphism phi from the
concordance group C_n of knotted (2n-1)-spheres in S^{2n+1} to an algebraically
defined group G_{+-}; furthermore, G__{+-} is isomorphic to the infinite direct
sum Z^infty direct sum Z_2^infty direct sum Z_4^infty. It was a startling
consequence of the work of Casson and Gordon that in the classical case the
kernel of phi on C_1 is infinitely generated. Beyond this, little has been
discovered about the pair (C_1,phi).
  In this paper we present a new approach to studying C_1 by introducing a
group, M, defined as the quotient of the set of knots by the equivalence
relation generated by concordance and positive mutation, with group operation
induced by connected sum. We prove there is a factorization of phi,
C_1-->M-->G_-. Our main result is that both maps have infinitely generated
kernels.
  Among geometric constructions on classical knots, the most subtle is positive
mutation. Positive mutants are indistinguishable using classical abelian knot
invariants as well as by such modern invariants as the Jones, Homfly or
Kauffman polynomials. Distinguishing positive mutants up to concordance is a
far more difficult problem; only one example has been known until now. The
results in this paper provide, among other results, the first infinite families
of knots that are distinct from their positive mutants, even up to concordance.
