In this task, you are given a sentence. You are expected to recognize the name of gene or protein. Although there might be several correct answers, you need to write one of them.

The role of the chemokine binding stromal - derived factor 1 ( SDF - 1 ) in normal human megakaryopoiesis at the cellular and molecular levels and its comparison with that of thrombopoietin ( TPO ) have not been determined .
SDF - 1