A major epitope of GPI-specific CD4+ T cells in glucose-6-phosphate isomerase-induced arthritis (GIA) was identified as peptide 325–339 glucose-6-phosphate isomerase (pGPI), and immunization with pGPI can also induce arthritis just as GIA does in DBA/1 mice (peptide 325–339 glucose-6-phosphate isomerase-induced arthritis [pGIA]) [11]. Here, CD4 is linked to arthritic joint disease.