Source:http://linkedlifedata.com/resource/umls/id/C0033991
MSH: Compounds based on pyrazino[2,3-d]pyrimidine which is a pyrimidine fused to a pyrazine, containing four NITROGEN atoms.,CSP: 10-membered bicyclic aromatic ring structure with 4 nitrogen heteroatoms at the 1,3,6,9 positions, giving the appearance of fused pyrimidine and pyrazine rings; 2-amino-4-keto derivative is characteristic of the folacins and other pterins.