Update README.md

README.md

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 ## Data description
 
 DNA methylation per site can be reported in two ways: as **beta values**, or as **M values**. Briefly, beta values represent the ratio of methylated signal 
-(probe intensity or sequencing read counts) to total signal per site (methylation signal is measured as probe intensity or sequencing read counts), 
+(probe intensity or sequencing read counts) to total signal per site (sum of methylated and unmethylated probe intensities or sequencing read counts), 
 while M value is the log2 ratio of the methylated signal versus an unmethylated signal. A more thorough explanation can be found in 
 [Du et al., 2010](https://doi.org/10.1186/1471-2105-11-587).