As the administration of Cys2 variant conjugated with 40 kDa PEG has resulted in much longer biological activity, we checked whether the administration of the Cys2 40 kDa PEG conjugated variant at the day of neutropenia induction (mimicking the G-CSF treatment at the day of chemotherapy) could facilitate the recovery of leukocyte count in a neutropenic mice model. Here, CSF3 is linked to Decreased total neutrophil count.