The mucin family is involved in various aspects of cancer progression, such as cell adhesion, epithelial-mesenchymal transition, cell signaling, and the tumor microenvironment. Mucins either alone or through interactions with receptor tyrosine kinases have been shown to mediate cell signals for the growth and survival of lung cancer cells. Some mucins, MUC1 (CA15-3) and MUC16 (CA125), are useful biomarkers to monitor malignant neoplasms. MUC21 was first identified as a human homologue of mouse Muc21/epiglycanin from human cervical carcinoma cell lines. MUC21 is a transmembrane mucin and is expressed in various human neoplasms including lung carcinomas and is linked with aggressive behavior of neoplastic cells. MUC21 could be a promising biomarker with potential diagnostic and therapeutic applications for lung adenocarcinoma showing cell incohesiveness.