had been mutating
- past perfect progressive of mutate.
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Trevor Bedford, a biostatistician at the Fred Hutchinson Cancer Research Center, said the coronavirus had been mutating at several times the rate of the flu virus, for which vaccines are redesigned annually.
From New York Times • Apr. 6, 2022