The present invention relates to one or more nucleic acid(s) encoding a binding molecule specifically binding to a human CD28 molecule, comprising (a) a nucleic acid sequence encoding a VH region and a nucleic acid sequence encoding a VL region comprising CDRs in a human immunoglobulin framework, wherein (i) the CDRs of the VH region (CDR-H) comprise the amino acid sequences of SEQ ID NOS: 2 or 18 (CDR-H3), 4 or 20 (CDR-H2) and 6 or 22 (CDR-H1) or are encoded by the nucleic acid sequences of SEQ ID NOS: 1 or 17 (CDR-H3), 3 or 19 (CDR-H2) and 5 or 21 (CDR-H1); and (ii) the CDRs of the VL region (CDR-L) comprise the amino acid sequences of SEQ ID NOS: 8 or 24 (CDR-L3), 10 or 26 (CDR-L2) and 12 or 28 (CDR-L1) or are encoded by the nucleic acid sequences of SEQ ID NOS: 7 or 23 (CDR-L3), 9 or 25 (CDR-L2) and 11 or 27 (CDR-L1); and (b) a nucleic acid sequence encoding the constant region of a human IgG1 or IgG4 antibody.