Sentence:  Double <WSD>ambers</WSD> revert at 10-8-10-9, and therefore, reversion is negligible. Double-amber mutants are made by crossing single-amber mutants with each other. 
Meaning of the word marked with <WSD> tags:  The stop codon (nucleotide triplet) "UAG", or a mutant which has this stop codon at a premature place in its DNA sequence.