Cloning efficiency of ELM cells repeatedly cloned in SCF or IGF-1. ELM cells cloned on stroma (30 clones) were cloned in 10 ng/mL SCF, 100 ng/mL IGF-1, or no growth factors and the cloning efficiency measured. Five clones surviving in each condition were then picked and recloned again in the same growth factors. In the case of cells cloned twice in SCF, the cloning in SCF or IGF-1 or no growth factors was repeated a third time.