Involved in alveolar fluid homeostasis via regulation of alveolar epithelial tight junction composition and therefore ion transport and solute permeability, potentially via downstream regulation of the actin cytoskeleton organization and beta-2-adrenergic signaling (By similarity). Required for lung alveolarization and maintenance of the paracellular alveolar epithelial barrier (By similarity). Acts to maintain epithelial progenitor cell proliferation and organ size, via regulation of YAP1 localization away from the nucleus and thereby restriction of YAP1 target gene transcription (By similarity). Acts as a negative regulator of RANKL-induced osteoclast differentiation, potentially via relocation of TJP2/ZO-2 away from the nucleus, subsequently involved in bone resorption in response to calcium deficiency (By similarity). Mediates the osteoprotective effects of estrogen, potentially via acting downstream of estrogen signaling independently of RANKL signaling pathways (By similarity). . [Isoform A1]: Involved in the maintenance of homeostasis of the alveolar microenvironment via regulation of pH and subsequent T- cell activation in the alveolar space, is therefore indirectly involved in limiting C.neoformans infection. . [Isoform A2]: Required for the formation of the gastric paracellular barrier via its role in tight junction formation, thereby involved in the response to gastric acidification. . This is the function of Ensembl gene identifier ENSG00000066405 (CLDN18, claudin 18).