The Knops blood group system consists of 9 antigens carried on a glycoprotein of 1998 amino acids and called the Complement Receptor I (CR1). It has a leader sequence of 41 to 46 amino acids, depending on translation initiation site, which is cleaved from the membrane protein. The coding sequence starts at nucleotide 28. The Helgson phenotype, initially thought to be a KN serologic null, may be more likely the result of lower level CR1 density and may also involve lack of a high prevalence KN antigen [Pham Transfusion 2010 50(7):1435-43].