NIH/3T3 cells stably expressing the indicated hPAC1 NullWT, NullTr406, and NullTr433 (A); the hPAC1 HopWT, HopTr406, and HopTr420 (B); HopTr416 and HopTr420 (C); and HopTr420A416, HopTrA417, HopTr420A418, and HopTr420A418A419 (D) were incubated alone or in the presence of increasing concentrations of PACAP-27. Data are shown as fold stimulation of total inositol phosphates compared with basal or unstimulated inositol phosphate turnover and represent the means of at least three experiments performed in triplicate. Similar levels of inositol phosphate stimulation were observed for five independent clonal cell lines for each mutant receptor.