Clonality detection by mRNA (exonic polymorphisms) and protein polymorphisms. (A) X-chromosome inactivation occurs early in embryonic development. (B) mRNA transcription occurs only from the active X chromosome (with some exceptions). (C) Analysis of peripheral blood mRNA of a female with polycythemia vera. The amplified products can be differentiated by sequence-specific polymerase chain reaction of cDNA followed by ligase reaction detection. bp indicates base pair standards; pos c., positive control; neg c., negative control (no mRNA added to reaction mixture); R, reticulocytes; P, platelets; M, monocytes; Mo, cultured macrophages; B, B lymphocytes; T, T lymphocytes; NK, natural killer cells. (D) G6PD A and B isoforms can be differentiated by electrophoretic mobility.