Click Selection () in Toolbar | Press and hold down the mouse button on the sequence where you want the selection to start | move the mouse to the end of the selection while holding the button | release the mouse button
Alternatively, you can search for a specific interval using the find function described in Find.
drag the edge of the selection (you can see the mouse cursor change to a horizontal arrow
or press and hold the Shift key while using the right and left arrow keys to adjust the right side of the selection.
If you wish to select the entire sequence:
double-click the sequence name to the left