CSN2 and pulmonary arterial hypertension: Furthermore, gene expression levels of β-casein and whey acidic protein (Csn2 and Wap), which are the major milk proteins, were significantly elevated in the mammary gland of PAH mice compared with those of WT mice at P19 (Fig. 3C), whereas there are no differences in expression levels between WT and PAH mice at P13 (Fig. 3B).