The SPG21 protein may act as a negative regulator in CD4-dependent T cell activation, suggesting its role in regulating immune responses. Its interaction with CD4 suggests a molecular link affecting CD4-mediated T cell activation pathways. SPG21 Protein, Human (sf9, His) is the recombinant human-derived SPG21 protein, expressed by Sf9 insect cells , with N-His labeled tag.