CALR mutants activate TpoR mutant R102P, which is traffic-deficient and is not localized at the cell surface. (A) Schematic representation of TpoR R102P with the position of the mutation and N-glycosylations in the extracellular domain. (B) Transcriptional activity of CALR mutants in the presence of TpoR R102P. Values shown represent the average of 3 independent experiments, each performed with 3 biological replicates ± standard error of the mean. **P < .01. Statistical analysis (jmp pro12) was performed by the nonparametric multiple comparisons Steel test with a control group. (C) Long-term (selection) proliferation assays for TpoR R102P in the presence of CALR mutants. (D) Cell surface localization of TpoR R102P in the presence of CALR mutants measured by flow cytometry. Flow cytometry analysis using an anti-human TpoR (CD110) of retrovirally transduced bone marrow cells from C57Bl6 mice expressing either TpoR R102P alone or together with CALR del52 (E), or bone marrow cell from KI-Calr^del52/WT mice transduced or not with TpoR R102P (F).