Filgrastim is a granulocyte colony-stimulating factor (G-CSF) analog used to stimulate the proliferation and differentiation of granulocytes; it is a pharmaceutical analog of naturally occurring G-CSF. It is produced by recombinant DNA technology. The gene for human granulocyte colony-stimulating factor is inserted into the genetic material of Escherichia coli. The G-CSF then produced by E. coli is different from G-CSF naturally made in humans.