  The level of isotopic fractionation in molecules provides insights into their
formation environments and how they formed. In this article, we review hydrogen
and nitrogen isotopic fractionation in low-mass star-forming regions.
Interstellar molecules are significantly enriched in deuterium. The importance
of the nuclear spin states of light species on deuterium fractionation and the
usefulness of singly and doubly deuterated molecules as chemical tracers are
discussed. Observations have revealed that molecules in prestellar cores are
enriched in or depleted in 15N depending on molecules. Compared with deuterium
fractionation chemistry, our understanding of 15N fractionation chemistry is
not well established. We briefly discuss potential 15N fractionation routes,
i.e., isotopic-exchange reactions and isotopic selective photodissociation of
N2. In addition, the selective freeze-out of 15N atoms onto dust grains around
the transition between N atoms and N2 is discussed as a potential mechanism
that causes the depletion of 15N in the gas phase.
