Biologic aspects of MPL W515L and MPL W515K mutations. (A) Whole blood mutant allele quantitation at trial entry showing significant difference in mean mutant allele burden for MPL W515L (mean mutant allele burden 17% ± 11%,) and MPL W515K (mean mutant allele burden 66% ± 26%, P < .001). (B) Genotyping of single erythroid colonies from 5 ET patients, 4 with MPL W515L and 1 with MPL S505N; a single homozygous colony was identified in patient 3.