A DNA sequence encoding the human/cynomolgus/rhesus CD28 (P10747-1/Q0PDN3-1/NP_001036106.2) (Asn19-Pro152) was expressed with a polyhistidine tag at the C-terminus and a signal peptide (Q0PDN3-1/NP_001036106.2, Met1-Gly18) at the N-terminus. Human, Cynomolgus and Rhesus CD28 sequences (Asn19-Pro152) are identical.