['cystic fibrosis']
['vx-661 plus ivacaftor combination', 'ivacaftor', 'placebo (matched to vx-661 plus ivacaftor combination)', 'placebo (matched to ivacaftor)']

        Inclusion Criteria:

          -  Confirmed diagnosis of CF defined as a sweat chloride value greater than or equal to
             (>=)60 millimole per liter (mmol/L) by quantitative pilocarpine iontophoresis.

          -  Heterozygous for the F508del-CFTR mutation and with a second CFTR mutation that is not
             likely to respond to VX-661 and/or ivacaftor therapy, genotype to be confirmed via
             assessment at the Screening Visit.

          -  Forced Expiratory Volume in 1 Second (FEV1) >=40 percent (%) and less than or equal to
             (<=)90% of predicted normal for age, sex, and height at Screening Visit.

        Exclusion Criteria:

          -  History of any comorbidity that, in the opinion of the investigator, might confound
             the results of the study or pose an additional risk in administering study drug to the
             participant.

          -  An acute upper or lower respiratory infection, pulmonary exacerbation, or changes in
             therapy (including antibiotics) for pulmonary disease within 28 days before Day 1
             (first dose of study drug).

          -  History of solid organ or hematological transplantation.

          -  Ongoing or prior participation in an investigational drug study or use of commercially
             available CFTR modulator within 30 days of screening.

          -  Pregnant or nursing females.
      