Recombinant human CD28 (CD28 molecule), also known as TP44 (T-cell-specific surface glycoprotein CD28), encompassing amino acids 19-152. The C-terminus of this construct is fused with the Fc fragment of a human IgG1 followed by an Avi-Tag™. The recombinant protein was enzymatically biotinylated using the Avi-Tag™ and affinity purified.