P4HA1 and breast cancer: Our results show that the activity of the Hif1a regulon in the S1 cells of the 6Ho mice is significantly lower than that in 5Ht mice (adjusted p value = 0.01), again consistent with our previous study showing that upregulation of P4HA1 in aggressive breast cancers leads to the increased level of HIF-1α in the mammary tissues of the patients [5].