A Hamming space Lambda(n) consists of all sequences of length n over an alphabet Lambda and is endowed with the Hamming distance. In particular, any set of aligned DNA sequences of fixed length constitutes a subspace of a Hamming space with respect to mismatch distance. The quasi-median operation returns for any three sequences u, v, w the sequence which in each coordinate attains either the majority coordinate from u, v, w or else (in the case of a tie) the coordinate of the first entry, u; for a subset of Lambda(n) the iterative application of this operation stabilizes in its quasi-median hull. We show that for every finite tree interconnecting a given subset X of Lambda(n) there exists a shortest realization within Lambda(n) for which all interior nodes belong to the quasi-median hull of X. Hence the quasi-median hull serves as a Steiner hull for the Steiner problem in Hamming space. (C) 2008 Elsevier B.V. All rights reserved.