[en] The third phase of the Borexino experiment that's referred to as SOX is devoted to test the hypothesis of the existence of one (or more) sterile neutrinos at a short baseline (∼ 5-10 m). The experimental measurement will be made with artificial sources namely with a ¹⁴⁴Ce-¹⁴⁴Pr antineutrino source at the first stage (CeSOX) and possibly with a ⁵¹Cr neutrino source at the second one. The fixed ¹⁴⁴Ce-¹⁴⁴Pr sample will be placed beneath the detector in a special pit and the initial activity will be about 100-150 kCi. The start of data taking is scheduled for April 2018. The article gives a short description of the preparation for the first stage and shows the expected sensitivity. (authors)