Pyruvate kinase (PK) deficiency of erythrocytes (OMIM 266200) is the most common cause of hereditary non-spherocytic haemolytic anaemia due to defective glycolysis (Hirono et al, 2001). PK is encoded by two genes, PKLR and PKM, expressing four isozymes in mammalian tissues: L-type (hepatic) and R-type (erythrocytic) encoded by PKLR and M1- (muscle and brain) and M2- (fetal and most adult tissues) types encoded by PKM. The disorder is transmitted as an autosomal recessive trait. So far, at least 133 different mutations have been identified in PK deficiency (Bianchi & Zanella, 2000). During the last few years several homozygous PK null mutations have been identified, one large deletion, two one base deletions and one transition at a splice site (Zanella & Bianchi, 2000). To date, no homozygous nonsense mutation has been reported. In the severely affected large Swiss kindred reported in this study, we found, for the first time, a homozygous nonsense mutation (1318G→T) in PK deficiency. Twenty members of a large Swiss kindred were investigated. Routine haematological studies were performed using standard methods. PK deficiency was diagnosed by measuring the red blood cell activity according to the International Council for Standardization in Haematology. Genomic DNA was extracted by standard procedures using the Qiagen QIAmp®DNA Blood Mini Kit according to the manufacturers guidelines (Qiagen, Hilden, Germany). The entire coding region of the PKLR gene was amplified by polymerase chain reaction (PCR) as described (Baronciani et al, 1995). Single-strand conformation polymorphism (SSCP) analysis was performed according to Budowle et al (1991). and fragments were sequenced on a ABI Prism 310 Sequencer using the ABI Prism dRhodamine Dye Terminator Cycle Sequencing Ready Reaction Kit according to the manufacturers guidelines (Applied Biosystems). After PK-deficiency was discovered in one patient, extensive genetic information was ascertained resulting in the identification of a large pedigree with complex consanguinity (Fig 1). All 12 known patients were diagnosed as a result of life-threatening neonatal anaemia and jaundice. Five died within 2 days of birth the haemoglobin concentration at birth was between 3·9 and 11·2 g/dl (mean 7·5 g/dl). All seven surviving individuals required multiple blood transfusions (75–197ml/kg/year; mean 137 ml/kg/year) and were splenectomized later in childhood. One patient died at the age of 11 years because of septicemia after splenectomy. Large Swiss pedigree with complex consanguinity. All 12 known patients with a homozygous pyruvate kinase deficiency are indicated in black. Molecular diagnosis was performed in all numbered individuals (VII-1, VII-2,…). Confirmed heterozygotes are indicated as half black, half white symbols. The PK activity of all investigated family members is noted below the identification number (normal range 10–21 IE/gHb). One other distant relative, not included in this pedigree, was shown to be heterozygous. Blood was obtained of four affected and 16 non-affected members of this large family. By DNA sequence analysis of PKLR, the four clinically affected individuals were found to be homozygous for the nonsense mutation (1318G→T) which was named PK Aarau, according to the recommendations of the International Committee for Standardization in Hematology. The mutation causes a premature termination of translation resulting in a truncated protein lacking a terminal fragment of 33 amino acids. In addition, among the 20 family members tested, 14 were heterozygous and two individuals were wild type. The nonsense mutation (1318G→T) was confirmed by Restriction Fragment Length Polymorphism after digestion of the PCR product with BseRI. So far only four homozygous PK null mutations resulting in a truncated protein have been described (Bianchi & Zanelli, 2000). Although we were not able to establish directly the ‘null’ expression of this variant, the omission of the C-domain of the PK protein surely prevents the formation of significant quantities of active enzyme, since it is known that the C-domain contains the binding site for the allosteric effector fructose-1,6-diphosphate (F-1,6-DP) and decisive intersubunit and interdomain contact regions. The survival of patients with this PK variant raises the question of whether the PKLR-gene is indispensable and leads us to speculate that the persistence of the M2 enzyme ameliorated the PK deficiency in our patients. Further studies are needed to correlate M2-PK expression in the erythrocytes of patients with PK-deficiency, with severity of haemolysis.