P^k and P1 antigen expression on RBCs measured by flow cytometric analysis. P1 antigen expression is shown top left with a representative histogram and bottom left the mean fluorescence intensity (MFI) values in a bar graph. P^k antigen expression is shown to the right. In both cases, the genotypes of the tested cells are indicated below the x-axis. As indicated above the histograms, the P¹P¹ sample is shown with a dark bold line, the heterozygous P¹P² sample with a gray solid line, and the P²P² sample is shown by a dotted line. The pp sample (negative control) is shown as a filled gray peak, and for the P^k expression, a P₁^k sample (positive control) is included and shown with a filled black peak.