  We study an evolving bipolar active region that exhibits flux cancellation at
the internal polarity inversion line, the formation of a soft X-ray sigmoid
along the inversion line and a coronal mass ejection. The evolution of the
photospheric magnetic field is described and used to estimate how much flux is
reconnected into the flux rope. About one third of the active region flux
cancels at the internal polarity inversion line in the 2.5~days leading up to
the eruption. In this period, the coronal structure evolves from a weakly to a
highly sheared arcade and then to a sigmoid that crosses the inversion line in
the inverse direction. These properties suggest that a flux rope has formed
prior to the eruption. The amount of cancellation implies that up to 60% of the
active region flux could be in the body of the flux rope. We point out that
only part of the cancellation contributes to the flux in the rope if the arcade
is only weakly sheared, as in the first part of the evolution. This reduces the
estimated flux in the rope to $\sim\!30%$ or less of the active region flux. We
suggest that the remaining discrepancy between our estimate and the limiting
value of $\sim\!10%$ of the active region flux, obtained previously by the flux
rope insertion method, results from the incomplete coherence of the flux rope,
due to nonuniform cancellation along the polarity inversion line. A hot linear
feature is observed in the active region which rises as part of the eruption
and then likely traces out field lines close to the axis of the flux rope. The
flux cancellation and changing magnetic connections at one end of this feature
suggest that the flux rope reaches coherence by reconnection shortly before and
early in the impulsive phase of the associated flare. The sigmoid is destroyed
in the eruption but reforms within a few hours after a moderate amount of
further cancellation has occurred.
