title: select elements of a dataframe according to the category of a row or column in R
body: <p>For each of the 4 genes(each gene is on column), i need to test whether its mean expression is equal for patients with stable and progressive disease and store the corresponding p-value. Someone can help me please ? the language is in R.
  Here picture of my dataframe:</p>
  
  <p><img src="https://i.stack.imgur.com/Fm1aN.png" alt="dataframe"></p>