Abstract: The present invention is based on the advantageous use of single-stranded oligonucleotides in the in vivo (within a cell) assembly of double-stranded oligonucleotides into a single double-stranded nucleic acid construct. The present invention relates to the use of at least a first and a second single-stranded oligonucleotide in the assembly within a cell of at least two double-stranded nucleic acid molecules into a single double-stranded nucleic acid construct of pre-determined sequence, wherein the first and second single-stranded oligonucleotide are essentially complementary to each other.