The study of erythrocyte membrane protein concentration by polyacrylamide gel electrophoresis (PAGE) is the first step in approaching the primary molecular defect in hereditary spherocytosis (HS). Normal or greater than normal protein 2.1 levels were found in ten unrelated HS patients showing the inactivation of one ankyrin allele. Erythrocyte membranes from the same patients, once splenectomized, showed a homogeneous degree of protein 2.1 reduction. Thus protein 2.1 levels could misleadingly appear normal due to the high number of circulating reticulocytes. To calculate the true ankyrin level using PAGE and consequently to avoid mistakes in studying a mutated gene, a simple equation, based on the number of reticulocytes, was developed.