  M2 branes suspended between adjacent parallel M5 branes lead to light
strings, the `M-strings'. In this paper we compute the elliptic genus of
M-strings, twisted by maximally allowed symmetries that preserve 2d (2,0)
supersymmetry. In a codimension one subspace of parameters this reduces to the
elliptic genus of the (4,4) supersymmetric A_{n-1} quiver theory in 2d. We
contrast the elliptic genus of N M-strings with the (4,4) sigma model on the
N-fold symmetric product of R^4. For N=1 they are the same, but for N>1 they
are close, but not identical. Instead the elliptic genus of (4,4) N M-strings
is the same as the elliptic genus of (4,0) sigma models on the N-fold symmetric
product of R^4, but where the right-moving fermions couple to a modification of
the tangent bundle. This construction arises from a dual A_{n-1} quiver 6d
gauge theory with U(1) gauge groups. Moreover we compute the elliptic genus of
domain walls which separate different numbers of M2 branes on the two sides of
the wall.
