It is my great pleasure and privilege to chair the first session of the 2nd Munich Advent Symposium on Purine and Pyrimidine Metabolism. This is a field in which the work of the Medical Polyclinic of the University of Munich is well known and highly regarded worldwide. The two inborn errors of metabolism involving the purine salvage metabolic pathways are hypoxanthine guanine phosphoribosyltransferase (HGPRT) deficiency and adenine phosphoribosyltransferase (APRT) deficiency. The range of clinical phenotypes associated with HGPRT deficiency is considerably wider than that observed in APRTdeficiency. Aconsiderable number of different qenomic lesions have now been identified in patients with HGPRT deficiency. These comprise the usual range of point mutations, single base deletions with frameshifts, small and large deletions, and insertions. It has not proved possible, so far, to clearly correlate these changes with either the clinical phenotypes or with the substrate binding sites on the enzyme molecule.