  The heavy quarks produced in the early stage of heavy-ion collisions are very
effective probes of the dense partonic medium produced at RHIC. PHENIX has the
ability to measure heavy quark production through single electrons in the
central arm spectrometers ($|\eta|<0.35$) and single muons in the forward
(backward) muon spectrometers ($1.2<|\eta|<2.2$). As these single leptons are
from open heavy-flavor meson semi-leptonic decays, initial state cold nuclear
matter effects on heavy quark production can be probed by measuring the single
leptons in $d$$+$Au collisions. PHENIX have observed a large enhancement of
heavy-flavor electrons in $d$$+$Au collisions at mid-rapidity, which indicates
strong CNM effects on heavy quark production, in contrast to the suppression
observed in Au$+$Au collisions. Measurement of single muons from open heavy
flavor in $d$$+$Au collisions at forward (backward) rapidity provide detailed
look into rapidity dependent CNM effects as well as the low (high) $x$ parton
distribution function within Au nucleus. We discuss recent PHENIX heavy flavor
measurements and how they expand our understanding of CNM effects and
contribute to the interpretation of other results in heavy-ion collisions.
