title: Remove rows in which string contains other letters than A,C,T,G,N
body: <p>I'm fairly new to <code>numpy</code> and <code>pandas</code>, let's say that I have a 2D numpy array and I need to delete all rows in which the second value contain only the letters <code>'A'</code>, <code>'C'</code>, <code>'T'</code>, <code>'G'</code> and <code>'N'</code></p>
  <pre><code>file = 
  [['id' 'genome'],
   ['0' 'ATGTTTGTTTTT'],
   ['1' 'ATGTTTGTXXXX'],
   ['2' 'ATGDD2GTTTTT']
  ]
  </code></pre>
  <p>so after filtering I can get this</p>
  <pre><code>[['id' 'genome'],
   ['0' 'ATGTTTGTTTTT']]
  </code></pre>
  <p>I wanted to do 3 for loops that are checking each char one by one but this is sooo slow when I have 500 rows</p>