A system for synthesizing a DNA molecule of user-defined sequence. A pre-defined, double-stranded, sequence of DNA with a single-stranded overhang is provided. The pre-defined, double-stranded, sequence of DNA is lengthened by the addition user-selected, single stranded DNA sequences.
 The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.