Mutation analysis of sorted GFP⁺ C-GFP clones. Sorted GFP⁺ cells were cultured for 1 wk, and 736 bp of the eGFP gene were cloned for sequencing analyses. From a total of 141 sequences, 121 contained a mutation at the G nucleotide (bp 1,099) of the premature stop codon. The x and y axes indicate the basepair of the transgene and the number of mutations, respectively. These were mostly G-to-C transversions. GFP⁺ cells were selected after either 2 or 4 wk of culture.