Time constants of the first phase (A) and the second phase (B) of the PATP-induced current increase for WT-CFTR channels and mutations in NBD2’s signature sequence. Although the time constants of the first phase for all mutants tested are similar to that of WT-CFTR, the time constants for the second phase vary greatly among different mutant channels. For some mutants, the data numbers for the first and the second time constants are different. This is due to loss of the patch during recordings of the second phase. *, P < 0.01 when compared with WT-CFTR.