Find the lengths of the labelled pieces by electrophoresis. This tells you the positions of the particular nucleotide in the original fragments. Fragments from double splits will not be labelled. Repeating for other nucleotides gets the sequence of the fragments. Use pattern recognition software to work out how the fragments go together in the original strands.
Electrophoretic resolution limits length of fragments.
Make a protein normally made by a different organism. For example human proteins in a micro-organism. Many important proteins now made this way. Increases red blood cells in anaemic kidney dialysis patients. To counter growth deficiency in children.