In our efforts to understand the therapeutic effect of CCH in the Ndufs4 knockout mice, we discovered that the partial pressure of oxygen in the brain of Ndufs4 knockout mice (measured in the hypothalamus and vestibular nuclei) was 60 mmHg in 21% oxygen, compared to 30 mmHg in wildtype mice (7). This evidence concerns the gene NDUFS4 and columnar cell hyperplasia of the breast.