BHLHB3 is a basic helix-loop-helix (bHLH) domain-containing protein that acts as 
a transcriptional repressor. We found that BHLHB3 transcript levels were low in 
three human lung cancer cell lines and downregulated in human lung 
adenocarcinomas as compared to normal lung tissue. BHLHB3 gene overexpression 
inhibited colony formation of A549, NCI-H520 and NCI-H596 lung cancer cells. The 
reduced colony growth was likely due to inhibition of cell proliferation as 
suggested by the downregulation of cyclin D1 (CCND1) expression in NCI-H520 
cells transfected to overexpress the BHLHB3 gene; no evidence of apoptosis was 
observed. These results point to the potential role of the BHLHB3 protein as a 
tumor suppressor for lung cancer.