The smallness of neutrino masses provides a tantalizing allusion to physics beyond the standard model. Heavy neutral leptons (HNL), such as hypothetical sterile neutrinos, accommodate a way to explain this observation, through the see-saw mechanism. If they exist, HNL could also provide answers about the dark matter nature, and baryon asymmetry of the universe. Searches for the production of HNL at the LHC, in final states with three leptons or two leptons and jets, are presented. The sample of pp collisions collected by the CMS detector throughout 2016 is used, amounting to a volume of $35.6\,\mathrm{fb}^{-1}$.