NTRK1 and idiopathic pulmonary fibrosis: The mRNA expression levels of MERTK, CD36 (both macrophage apoptotic cell recognition receptors) and Rac1 (an indicator of cytoskeletal rearrangement for phagocytosis) were increased in IPF patients, while the mRNA level of Axl (a receptor tyrosine kinase from the TAM family) was not significantly different between the two groups (Fig. 1D).