The cyclin-dependent kinase 4 (cdk4) inhibitor (p16INK4/MTS1/CDKN2) gene has 
been recently identified as a putative tumor suppressor gene because of the high 
frequency of homozygous deletion observed in numerous human tumor cell lines, 
including leukemias. However, results obtained from uncultured tumor samples 
have led to discussion of the relevance of these findings. Using reverse 
transcriptase polymerase chain reaction (RT-PCR) and Southern blot analysis, we 
have investigated p16INK4A gene at both RNA and genomic levels in various types 
of leukemias: acute myeloid leukemia (AML) (n = 23); acute lymphocytic leukemia 
(ALL) (n = 22) and B cell chronic lymphoproliferative disorders (CLPD) (n = 33). 
p16INK4A mRNA expression was not found in only 1/20 AML and 2/23 CLPD samples. 
Conversely, p16INK4A mRNA was not detected in 5/17 ALL cases, and intensity of 
PCR products were barely detectable in seven additional cases, possibly related 
to the contamination by normal cells in some cases. By Southern blotting, a 
homozygous deletion of p16INK4A gene was found in 6/17 ALL cases (35%) among 
which 4/6 were negative or weakly positive by RT-PCR assay. None of the five AML 
and 20 CLL samples studied had p16INK4A deletion. Sequence analysis of p16INK4A 
exon 2 did not show point mutation in two of these cases lacking mRNA 
expression. Our data provide further evidence that among hematological 
malignancies, ALL are the most likely to be associated with p16INK4A 
inactivation, mainly by homozygous gene deletion. Since most hematological 
malignancies-except ALL-are infrequently associated with p16INK4A and 
retinoblastoma (Rb) gene alteration it seems worthwhile to explore cdk4 and cdk6 
expression to determine whether or not the disruption of the 
p16INK4A/Rb/cdk4/cdk6 regulatory loop might play a role in their pathogenesis.