CD4 and infection: Still, based on sCD4, CD4-IgG2 (PRO 542), comprised of the D1D2 domain of CD4, showed high affinity [33], and CD4-IgG2 could neutralize more primary isolates than sCD4 alone [34], and it could protect the hu-PBL-SCID mouse from infection of primary HIV-1 isolates [35].