The invention relates to viral vectors comprising nucleic acid sequences coding for single chain interleukin-12 (single chain IL-12 or scIL12) and a costimulator protein, and to the use of vectors for gene therapy, especially for the treatment of tumors. The invention further relates to adenoviral vectors containing nucleic acid sequences having a sequence homology of at least 90% in relation to the sequence displayed in FIGS. 19 and 20 (IL-12), in FIG. 21 (4-1BB ligand) and in FIG. 22 (IL-2) and optionally also one of the sequences displayed in FIG. 23A (B7-1) or 23B (B7-2).