(a) DPOAE threshold (2f1–f2, 0 dB SPL threshold criterion, mean±s.d.) as a function of the f2 frequency (f2/f1 ratio=1.23; level of f2 set 10 dB below f1 level) from six CD-1Cx30WT/WT (blue symbols) and five CD-1Cx30A88V/A88V (black symbols) mice, and from seven CBA/J mice (red symbols). The maximum SPL of the sound system was restricted to ≤110 dB for frequencies ≥35 kHz. (b,c) DPOAE magnitude (2f1–f2, mean±s.d.) as a function of the f2 frequency for five CD-1Cx30A88V/A88V mice (b) and five CD-1Cx30WT/WT mice (c). Stimulus levels and colour and symbol coding for f1 and f2 are shown in the figures. X axis: f2 frequency (kHz) for all figures. Dashed and dotted lines indicate the recording noise floor±s.d. for all measurements.