Cell cycle dependent phosphorylation of the RB tumor suppressor protein is 
mediated by a family of G1 cyclin dependent kinases (cdks) and cyclins including 
the activated cdk4:cyclin D complex. The identification of a cdk4 inhibitor, 
p16INK4, as a target for mutations in cultured tumor lines and primary tumors 
suggested that RB activity may be affected in these cells. We have examined 88 
lung cancer lines for p16INK4 protein expression and have observed a striking 
inverse correlation between the presence of p16INK4 and wildtype RB. We 
demonstrated that only 6/55 (11%) of small cell lung cancer (SCLC) samples had 
absent p16INK4 protein, and all 6 belonged to the rare subset of SCLC with 
wildtype RB expression. Conversely of 48 SCLC samples with absent or mutant RB, 
all showed detectable levels of p16INK4 protein. In contrast, we observed that 
23/33 (70%) of non-SCLC samples had loss of p16INK4. Twenty-two of 26 non-SCLC 
lines with wildtype RB had absent p16INK4 while 6 of 7 non-SCLC lines with 
absent or mutant RB had detectable p16INK4. The inverse correlation of RB and 
p16INK4 expression and the absence of p16INK4 inactivation in RB (-/-) SCLC 
lines (0/48) confirms a common p16INK4/RB growth suppressor pathway in human 
cancers and provides evidence that p16INK4, and not an adjacent gene on 
chromosome 9p, is a specific target for mutational events.