BACKGROUND: Activating mutations in one allele of an oncogene (heterozygous 
mutations) are widely believed to be sufficient for tumorigenesis. However, 
mutant allele specific imbalance (MASI) has been observed in tumors and cell 
lines harboring mutations of oncogenes.
METHODOLOGY/PRINCIPAL FINDINGS: We determined 1) mutational status, 2) copy 
number gains (CNGs) and 3) relative ratio between mutant and wild type alleles 
of KRAS, BRAF, PIK3CA and EGFR genes by direct sequencing and quantitative PCR 
assay in over 400 human tumors, cell lines, and xenografts of lung, colorectal, 
and pancreatic cancers. Examination of a public database indicated that 
homozygous mutations of five oncogenes were frequent (20%) in 833 cell lines of 
12 tumor types. Our data indicated two major forms of MASI: 1) MASI with CNG, 
either complete or partial; and 2) MASI without CNG (uniparental disomy; UPD), 
due to complete loss of wild type allele. MASI was a frequent event in mutant 
EGFR (75%) and was due mainly to CNGs, while MASI, also frequent in mutant KRAS 
(58%), was mainly due to UPD. Mutant: wild type allelic ratios at the genomic 
level were precisely maintained after transcription. KRAS mutations or CNGs were 
significantly associated with increased ras GTPase activity, as measured by 
ELISA, and the two molecular changes were synergistic. Of 237 lung 
adenocarcinoma tumors, the small number with both KRAS mutation and CNG were 
associated with shortened survival.
CONCLUSIONS: MASI is frequently present in mutant EGFR and KRAS tumor cells, and 
is associated with increased mutant allele transcription and gene activity. The 
frequent finding of mutations, CNGs and MASI occurring together in tumor cells 
indicates that these three genetic alterations, acting together, may have a 
greater role in the development or maintenance of the malignant phenotype than 
any individual alteration.