C57BL/6 and BALB/c WT mice were obtained from The Jackson Laboratory. Standard targeting approaches and homologous recombination were used to generate BRP-39–null embryonic stem cells and knockout mice (Fig. S1). Mice with null mutations in BRP-39 were generated on a mixed 129/C57BL/6 background and subsequently bred for >10 generations onto a C57BL/6 or BALB/c background. CC10–rtTA–IL-13 Tg mice were generated in our laboratory (20 (link)), bred onto a C57BL/6 background, and used in these studies. These mice use the Clara cell 10-kD protein (CC10) promoter and the rtTA (reverse tetracycline transactivator) to target IL-13 to the lung in a dox-inducible manner. Tg mice in which human YKL-40 was tightly and inducibly overexpressed (CC10-rtTA-tTS-YKL-40) in a lung-specific manner were generated using constructs and approaches that have been previously described by our laboratory (Fig. S7) (42 (link)). Mice that lacked BRP-39 and produced YKL-40 only in pulmonary epithelial cells (CC10-rtTA-tTS-YKL-40/BRP-39−/−) were generated by breeding the CC10-rtTA-tTS-YKL-40 and BRP-39−/− mice. Animal protocols were approved by the Yale University Institutional Animal Care and Use Committee (IACUC), and all experiments were performed according to the guidelines of the Yale University IACUC.