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The separated vertices for charm mesons together with precise
vertex location,
excellent particle identification, and excellent 
reconstruction make
it possible to reduce backgrounds and study rare decay modes not accessible in
other experiments. One example is the doubly Cabibbo-suppressed decay
. Without good (
) particle identification this
process is not easily separated from the background process
.
Other rare decays provide a unique opportunity for looking beyond the
Standard Model. Flavor-changing neutral currents, manifested by the
decay 
, should exist and may have branching fractions of the
order of 
. Present limits by CLEO II are of the order of 
.