Circulating tumor DNA (ctDNA) is now being extensively studied as it is a 
noninvasive "real-time" biomarker that can provide diagnostic and prognostic 
information before, during treatment and at progression. These include DNA 
mutations, epigenetic alterations and other forms of tumor-specific 
abnormalities such as microsatellite instability (MSI) and loss of 
heterozygosity (LOH). ctDNA is of great value in the process of cancer 
treatment. However, up to date, there is no strict standard considering the 
exact biomarker because the development and progression of cancer is extremely 
complicated. Also, results of the studies evaluating ctDNA are not consistent 
due to the different detection methods and processing. The major challenge is 
still assay sensitivity and specificity for analysis of ctDNA. This review 
mainly focuses on the tumor specific DNA mutations, epigenetic alterations as 
well as detecting methods of ctDNA. The advantages and disadvantages will also 
be discussed.