Update README.md

README.md

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 The dataset is from [Deep generative models of genetic variation capture the effects of mutations](https://www.nature.com/articles/s41592-018-0138-4).
 
 ### Label
-Label means fitness score of each mutant amino acid sequence based on a deep mutation scanning experiment.
+Label means fitness score of each mutant amino acid sequence based on a deep mutation scanning experiment. 
+The effect of each mutation is expressed as the log frequency of observing each amino acid x at each position i in the selected versus the unselected population, 
+relative to wild type. Range from negative infinity to positive infinity. If the effect larger than 0 represents high fitness, smaller than 0 represents low fitness.