title: Find row(s) that contain specific letter
body: <p>Have a large file (csv) data which I'm trying to parse genes that contain specific letters
  Example table (the actual data is 5GB and much larger matrix)</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Cell_Index</th>
  <th>880246</th>
  <th>13694</th>
  <th>491094</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>ABCA7</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  </tr>
  <tr>
  <td>zyg11</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  </tr>
  <tr>
  <td>ABR</td>
  <td>1</td>
  <td>0</td>
  <td>1</td>
  </tr>
  <tr>
  <td>ACAP2</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  </tr>
  <tr>
  <td>mtycap</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  </tr>
  <tr>
  <td>zyg11</td>
  <td>1</td>
  <td>1</td>
  <td>0</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I'm trying to rows that contain the letter &quot;mt&quot; in the Cell_Index column.
  What I tried :</p>
  <pre><code>df = df.loc[df['Cell_Index'].str.startswith(&quot;mt&quot;, case=False)]
  </code></pre>
  <p>When I ran that code it gave me an error message KeyError: 'Cell_Index'</p>
  <p>Not sure what I did wrong here.....</p>