Acquired mutations in the juxtamembrane region of MPL (W515L or W515K), the receptor for thrombopoietin, have been reported in patients with primary essential thrombocythemia (ET) or primary myelofibrosis (PMF). The mutations were detected by the newly developed real-time quantitative PCR (RQ-PCR) with TaqMan MGB probes and followed by the sequencing analysis. DNA samples were from 343 Chinese adults with JAK2V617F mutation-negative chronic myeloproliferative disorders (cMPDs). Reference curves were obtained using cloned fragments of MPL containing either the wild-type or MPL W515L or MPL W515K mutated sequence; the predicted sensitivity level was at least 0.5%(0.1-0.5%) for MPL W515L and 0.5%(0.2-0.5%) for MPL W515K mutant allele in a wild-type background. The detection rates of MPL W515 mutations were 3.5% in 199 ET patients (7/199), 12.5% in 24 PMF patients (3/24) and 5.6% in 36 cMPD-unclassed patients (2/36), respectively. No MPL W515 mutations were detected in 32 polycythemia vera (PV) patients, 40 chronic myeloid leukaemia (CML) patients, 12 hypereosinophilic syndrome (HES) patients and 29 normal volunteers. The mean calculated burden of MPL mutant alleles using RQ-PCR for MPL W515L/K was 24.88 +/- 14.80% (range, 1.10-56.32%). MPL W515L/K patients presented lower haemoglobin levels, compared with the patients with JAK2V617F mutation-positive cMPDs (p < 0.01). The results demonstrated that RQ-PCR was a reliable and sensitive method for large-scale screening of the MPL W515L/K mutation in patients suspected to have a cMPD. It can also provide a quantitative estimate of mutant allele burden that might be useful for both patient prognosis and monitoring response to therapy.