Abstract: A method for preparing and isolating a transformation vector containing CSF/cDNA is described. The method comprises:preparing RNA from a cell that produces CSF;preparing polyadenylated messenger RNA from the RNA;preparing single stranded cDNA from the messenger RNA;converting the single stranded cDNA to double stranded cDNA;inserting the double stranded cDNA into transformation vectors and transforming bacteria with the vector to form colonies;picking pools of 200 to 500 colonies each and isolating plasmid DNA from each pool;transfecting the plasmid DNA into suitable host cells for expressing CSF protein;culturing the transfected cells and assaying the supernatant for CSF activity; andselecting CSF positive pools and screening the colonies used to make the pool to identify a colony having CSF activity. Also described are a cDNA coding for a protein having CSF activity (i.e.