CXCL12 and rheumatoid arthritis: To examine whether the interaction between RA EC HSPGs and CXCL12α requires the cluster of basic residues that comprise a putative HSPG-binding motif in the first amino-terminal β strand of CXCL12 and sulfated groups on HSPGs as previously observed in filter binding assays [18], we compared the binding of wild-type CXCL12α and 2/6 CXCL12α (K2427S mutation) to RA ECs.