While in the Sequence area, if you hold the left mouse button and move the cursor, you will activate the selection mode. By moving the cursor you can adjust the size of the selection.
Releasing the mouse button will result in exiting the selection mode.
The selection mode is available in the Sequence list and the Consensus area too. The difference between these areas and the Sequence area is that here you can add to selection the whole rows or columns respectively.
To cancel the selection, press the Esc key.