Abstract: The invention relates to use of the nitrate reductase gene as a selection marker in transformations of the antibiotic-producing species Penicillium chrysogenum and Acremonium chrysogenum. In particular it relates to a process for obtaining cells of Penicillium chrysogenum or Acremonium chrysogenum capable of expressing nitrate reductase from cells of the same species which are initially deficient in expression of the nitrate reductase gene (niaD-cells), which comprises introducing into said niaD-cells vector DNA including a marker gene coding for nitrate reductase operatively linked to a control sequence for expression of said gene within the selected host cells, followed by selection of cells thus transformed by their ability to grow on a suitable medium containing nitrate as the only source of nitrogen and to vector DNA for use in such a process.