We present ab initio calculations of an electronic structure and a topological invariant nu(0) of the Ge2Sb2Te5 compound. We have found that in the case of equiatomic composition of Ge/Sb layers the Ge2Sb2Te5 compound is the topological insulator. The nu(0) invariant does not depend on a specific location of Ge and Sb atoms in the mixed layers, and depends only on their concentration. The variation of the concentration in the Ge/Sb layers leads to a change of the topological invariant of the compound. (C) 2012 Elsevier B. V. All rights reserved.