The severity of breathing abnormalities in Pet-1−/− mice can be influenced by environmental conditions. (A,B,C) Comparison of inspiratory (TI) and expiratory (TE) time (A), breathing frequency (B) and the coefficients of variation of these ventilatory parameters (C) in P4.5 wild type (+/+, dark bars) and Pet-1 null mutant (−/−, light bars) mice reared in either a pathogen-exposed (PE) or pathogen-free (PF) environment. (D,E,F) Direct comparison of inspiratory (TI) and expiratory (TE) time (D), breathing frequency (E) and the coefficients of variation of these ventilatory parameters (F) in P4.5 +/+ mice from the PE and PF colonies. (G,H,I) Direct comparison of inspiratory (TI) and expiratory (TE) time (G), breathing frequency (H) and the coefficients of variation of these ventilatory parameters (I) in P4.5 Pet-1−/− mice from the PE and PF colonies. (J) Pet-1−/− mice from the PF colony also suffered an increased incidence of prolonged respiratory pauses, relative to wild type littermates (compare with Fig. 1C). Values are means ± SD. *P<0.05; **P<0.01. See Table 2B, Table 3 for sample sizes and P values.