Two single-nucleotide substitutions in PKLR constituted the molecular basis underlying pyruvate kinase (PK) deficiency in a patient with severe haemolytic anaemia. One novel mutation, IVS5+1G>A, abolished the intron 5 donor splice site. The other mutation, c.1436G>A, altered the intron 10 donor splice site consensus sequence and, moreover, encoded an R479H substitution. We studied the effects on PKLR pre-mRNA processing, using ex vivo-produced nucleated erythroid cells from the patient. Abolition of the intron 5 splice site initiated two events in the majority of transcripts: skipping of exon 5 or, surprisingly, simultaneous skipping of exon 5 and 6 (Delta5,6). Subcellular localization of transcripts suggested that no functional protein was produced by the IVS5+1A allele. The unusual Delta5,6 transcript suggests that efficient inclusion of exon 6 in wild-type PKLR mRNA depends on the presence of splice-enhancing elements in exon 5. The c.1436G>A mutation caused skipping of exon 10 but was mainly associated with a severe reduction in transcripts although these were, in general, normally processed. Accordingly, low amounts of PK were detected in nucleated erythroid cells of the patient, thus correlating with the patient's PK-deficient phenotype. Finally, several low-abundant transcripts were detected that represent the first examples of "leaky-splicing" in PKLR.