Data items in the PDBX_FAMILY_GROUP_INDEX category record the family membership in family groups.
loop_ _pdbx_family_group_index.id _pdbx_family_group_index.family_prd_id FGR_0000001 FAM_0000001 FGR_0000001 FAM_0000002 FGR_0000001 FAM_0000021 FGR_0000002 FAM_0000022 FGR_0000002 FAM_0000023 # .....