  Mass distribution of black holes in low-mass X-ray binaries previously
suggested the existence of a $ \sim 2-5M_{\odot} $ mass gap between the most
massive neutron stars and the least massive black holes, while some recent
evidence appears to support that this mass gap is being populated. Whether
there is a mass gap or not can potentially shed light on the physics of
supernova explosions that form neutron stars and black holes, although
significant mass accretion of neutron stars including binary mergers may lead
to the formation of mass-gap objects. In this review, I collect the compact
objects that are probable black holes with masses being in the gap. Most of
them are in binaries, their mass measurements are obviously subject to some
uncertainties. Current observations are still unable to confidently infer an
absence or presence of the mass gap. Ongoing and future surveys are expected to
build the mass spectrum of black holes which can be used to constrain the
process of their formation especially in binaries. I describe the theoretical
predictions for the formation of black holes in various types of binaries, and
present some prospects of searching for black holes via electromagnetic and
gravitational wave observations.
