mRNA expression analysis of RPK and M2PK by RT-PCR in peripheral mononuclear cells and red cells. An RPK-mRNA band was detected in red and mononuclear cell fractions from the patient and a control patient with hereditary spherocytosis, yet it was undetectable in fractions from a healthy control. The M2PK-mRNA band was only found in the mononuclear cell fraction of all 3 samples. β-Actin mRNA expression was used as an internal control for all the sample sets. It should be noted that PKLR mRNA from reticulocytes/erythroblasts, mainly from the patient and a control with hereditary spherocytosis, could be present in the mononuclear cell fraction.