  We present the detection of a 2.0(+1.4,-0.8)x10^4 solar mass black hole (BH)
in the stellar cluster G1 (Mayall II), based on data taken with the Space
Telescope Imaging Spectrograph onboard the Hubble Space Telescope. G1 is one of
the most massive stellar clusters in M31. The central velocity dispersion (25
kms) and the measured BH mass of G1 places it on a linear extrapolation of the
correlation between BH mass and bulge velocity dispersion established for
nearby galaxies. The detection of a BH in this low-mass stellar system suggests
that (1) the most likely candidates for seed massive BHs come from stellar
clusters, (2) there is a direct link between massive stellar clusters and
normal galaxies, and (3) the formation process of both bulges and massive
clusters is similar due to their concordance in the M_BH/sigma relation.
Globular clusters in our Galaxy should be searched for central BHs.
