Two retinoblastoma related proteins, p107 and pRb2 / p130, which are structurally and functionally similar to the product of the retinoblastoma gene (pRb / p105), were cloned by taking advantage of their ability to bind transforming proteins of DNA tumor viruses through a particular region called the "pocket domain." Like pRb, both proteins play a fundamental role in growth control. These Rb family proteins were measured in a variety of lung neoplasms. The highest percentage of undetectable levels and the tightest inverse correlation with the histological grading and with PCNA expression in the most aggressive tumor types were found for pRb2 / p130, which may suggest an important role for this protein in the pathogenesis and progression of lung cancer.