The present invention relates to the use of interleukin-15 (IL-15) as selectable marker for gene transfer, preferably of at least one gene of therapeutic interest, into a mammalian cell or cell line, in particular a human cell or cell line. The present invention furthermore relates to transgenic mammalian cells or cell lines expressing IL-15 as selectable marker and co-expressing at least one protein of interest encoded by at least one gene of interest, which is preferably a protein of therapeutic interest. The present invention is in particular suitable for chimeric antigen receptors (CARs) as the gene or protein of interest and their expression in lymphocytes. The transgenic mammalian cells and cell lines are furthermore suitable for use as a medicament, in particular in the treatment of cancer and in immunotherapy, such as adoptive, target-cell specific immunotherapy.
