Plot of the number of erythroid burst component colonies as a percentage of maximum versus the molar concentration of IGFBP-1 in the presence of 3 × 10⁻¹¹ mol/L IGF-I. Representative data obtained for cells of normal individuals (○) and PV patients (•) are shown. The half-maximum value for PV occurs at ∼1 × 10⁻¹⁴ mol/L IGFBP-1, whereas that for the normal occurs at ∼6 × 10⁻¹³ mol/L IGFBP-1. Thus, with respect to burst formation, erythroid progenitor cells in PV are approximately 60-fold more sensitive than normal to IGFBP-1 in the presence of IGF-I.