Summary.In nine unrelated Portuguese patients with pyruvate kinase (PK) deficient anaemia, whose symptoms ranged from a mild chronic haemolytic anaemia to a severe anaemia presenting at birth and requiring multiple transfusions, the PK-LR gene mutations were identified and correlated with their phenotypes. Five different mutations were identified, three of them for the first time: a missense mutation 1670G → C on exon 12 and two 5 0 splice donor site (GT) mutations on intron 8 [IVS8(þ2)T → G] and intron 10 [IVS10(þ1)G → C]. Two previously described missense mutations, 1456C → T and 993C → A, were also found. The genotype/phenotype correlation showed that patients with two missense mutations or with a missense mutation and a splicing mutation had a mild haemolytic anaemia. The three patients with severe anaemia, who were transfusion dependent until splenectomy, were homozygous for the splicing site mutations IVS10(þ1)G → C or IVS8(þ2)T → G.Keywords: pyruvate kinase deficiency, human PK-LR gene mutations, haemolytic anaemia, splice site, Portugal.Pyruvate kinase (PK EC 2.7.1.40) deficiency is the most common enzyme abnormality in the erythrocyte glycolytic pathway causing hereditary nonspherocytic haemolytic anaemia (HNSHA). The PK-deficient anaemia is transmitted as an autosomal recessive disorder with a heterogenous clinical phenotype, ranging from a mild chronic haemolytic anaemia to a severe anaemia presenting at birth and requiring exchange transfusion. Splenectomy is effective in decreasing haemolysis in most patients. Heterozygotes are usually asymptomatic, with a 30-50% decrease in the PK activity levels (Miwa et al, 1979). PK deficiency was first described by Valentine et al (1961) and since then more than 400 cases have been reported, with the biochemical characteristics of the residual enzyme differing markedly between patients. The International Committee for Standardization in Haematology (ICSH) established the standard methods for enzymatic studies; however, the low activity, different stability and the hybrid tetramers formed in the compound heterozygous causes difficulty in characterization of the PK variants (Miwa et al, 1979). A molecular approach, identifying the gene mutations, enables a better characterization of the PK-deficient patients with a more precise genotype/phenotype correlation.It is also useful for diagnosis among transfusion-dependent patients with chronic haemolytic anaemia and prenatal diagnosis.Four PK isoenzymes have been described in mammals, M1, M2, L and R, which are tissue specific and differ in their enzymatic properties (Imamura & Tanaka, 1972). In mammals there are only two different PK genes: the PK-M gene that codes for the isoenzymes M1 (muscle and brain) and M2 (fetal and most adult tissues) using alternative RNA splicing (Noguchi et al, 1986) and the PK-LR gene, that codes for the L-type (liver) and R-type (erythrocyte) isoenzymes using two different tissue-specific promoters (Noguchi et al, 1987). More than 70 mutations have been identified in the coding and flank...