Furthermore, the evidences such as the unique adaptive immune resistance mechanism emerged from CD44 expressing cells in squamous cell carcinoma (SCC) and positive correlation of CD44 expressing cell number with the tumor progression and relapse in SCLC support the potential role of CD44 expressing SCLC cells in tumor progression by developing adaptive resistance to immune destruction [23, 44]. This evidence concerns the gene CD44 and small cell lung carcinoma.