Based on the work of Hoogaars [24,50], a fall of Tbx3 (as observed in the sinus node during PAH—figure 3c) would be expected to result in a fall in HCN1 and HCN4 and figure 3a,b shows that such a fall was observed and these are clues concerning the driving force behind the remodelling of the sinus node. This evidence concerns the gene HCN1 and pulmonary arterial hypertension.