The invention pertains to the field of adaptive cell immunotherapy. It aims at reducing the occurrence of translocations and cell deaths when several specific endonuclease reagents are used altogether to genetically modify primary immune cells at different genetic loci. The method of the invention allows to yield safer immune primary cells harboring several genetic modifications, such as triple or quadruple gene inactivated cells, from populations or sub-populations of cells originating from a single donor or patient, for their subsequent use in therapeutic treatments.