Add internal modifications: When applicable, choose your internal modification by clicking on the black triangle ( ) related to the base to modify.
Modified bases are displayed as a plain dark box ( ).
X units ( ) must always have an internal modification.
Choose a linkage: By default bases are linked by a phosphodiester bond ().
Clicking on the link between to bases changes it to a phosphorothioate linkage (yellow) (). A * is then added to the sequence.