Lung cancer is known to carry multiple cytogenetic alterations. Among such chromosomal abnormalities, deletions at the short arm of chromosome 3 has been shown to be the most frequent and can be detected even in hyperplasia of the bronchial epithelium, suggesting its important roles in the pathogenesis of lung cancer. In this chapter, we describe current status of the positional-cloning attempts to identify the target tumor suppressor gene at 3p in our laboratories as well as in other laboratories.