Tumor EpoR Expression and Clinical Outcomes. Recent studies have established that EpoR mRNA and protein levels vary substantially among tumor samples from different patients. Model A: EpoR levels are not sufficient to bestow Epo responsiveness. Model B: Adverse effects of Epo in cancer patients are mediated via direct effects on tumor cells in a manner that depends upon tumor EpoR levels. Clinically focused studies using primary patient tumor specimens are needed to discriminate between these two models.