BMPR2 and pulmonary arterial hypertension: The absence of pulmonary hypertension at baseline in mice carrying heterozygous Bmpr2 mutations (with mutant Bmpr2 alleles expressed at levels similar to those of the WT allele) is consistent with the observation that PAH occurs in only one-fifth of the individuals harboring BMPR2 mutations, suggesting that additional genetic or environmental factors (second hit) are involved in the clinical manifestation of the disease [14].