Initially , the ability of cells to survive and proliferate from a single cell - an important characteristic of self - renewing cells - was evaluated by a colony - formation assay . All three clones ( ASTcl3 , ASTcl6 , and ASTcl7 ) acquired the ability to form colonies , as evidenced after 5 days of plating ( Figure 2B ) . ASTcl7 formed more colonies compared with ASTcl3 and ASTcl6 ( Figure 2C ) . Notably , primary astrocytes were also analyzed , and proved unable to form colonies ( data not shown ) .