Effect of an IGF-RI blocking antibody on the induction of DNA-synthesis by IGF-I and E2 in the three MCF-7 strains. Serum-starved cells were grown in 24-wells plates, and were preincubated for 1 hour with 1 μg/ml of αIR3 antibody before stimulation with mitogens. Cells were treated with no mitogens (-), 20 ng/ml of IGF-I (I(20)), 2 ng/ml of IGF-I (I(2)), 2 ng/ml of IGF-I in combination with 1 nM E2 (I(2)E2), or 1 nM E2 (E2) and were harvested after 30 h. 3H-thymidine (2 μCi/ml) was added to the culture 6 h before harvesting. Indicated values are means of triplicate measurements. The values were normalized for the incorporation induced by 20 ng/ml of IGF-I (100%).