Using a kinetic reversible 2-compartment 4k (2T4k) model of the lung 18FLT PET, Ali Ashek et al. showed an increased rate of 18FLT (a marker for cell proliferation) phosphorylation, k3, together with prominent thymidine kinase 1 (TK1) immunostaining in the remodeled PAs of IPAH patient lungs [67]. Here, TK1 is linked to idiopathic pulmonary arterial hypertension.