Sentence:  For example, to cross a temperature-sensitive mutation with an <WSD>amber</WSD> mutation, <WSD>amber</WSD> suppressor cells are infected at the low (permissive) temperature. 
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.