To examine whether the secreted GM-CSF was biologically active, GM-CSF-dependent FDCP-1 cells were labeled with CFSE and consequently cultured in the presence or absence of recombinant murine GM-CSF, as well as in conditioned medium (CM) of CT26-GM-CSF and CM of non-modified CT26 tumor cells. This evidence concerns the gene CSF2 and neoplasm.