Ca²⁺ influx after application of Hcrt-1 using wild type (A) and E54K (B) receptors (for technical details, see Methods). Ionomycin induces similar calcium influx in control and mutated receptors. Control receptors are activated at a higher level and by significantly lower Hcrt-1 concentrations (A) than the mutated E54K receptor. Similar results were obtained with Hcrt-2 (data not shown). Truncated mutants (Labrador and Doberman) did not react to Hcrt stimulation (see text).