  For a triangulated category T, if C is a cluster-tilting subcategory of T,
then the quotient category T\C is an abelian category. Under certain
conditions, the converse also holds. This is an very important result of
cluster-tilting theory, due to Koenig-Zhu and Beligiannis.
  Now let B be a suitable extriangulated category, which is a simultaneous
generalization of triangulated categories and exact categories. We introduce
the notion of pre-cluster tilting subcategory C of B, which is a generalization
of cluster tilting subcategory. We show that C is cluster tilting if and only
if B/C is abelian.
