  Let X be a Mumford-Tate variety, i.e., a quotient of a Mumford-Tate domain D
by a discrete subgroup. Mumford-Tate varieties are generalizations of Shimura
varieties. We define the notion of a special subvariety Y in X (of Shimura
type), and formulate necessary criteria for Y to be special. Our method
consists in looking at finitely many compactified special curves C_i in Y, and
testing whether the inclusion of the union of all C_i in Y satisfies certain
properties. One of them is the so-called relative proportionality condition. In
this paper, we give a new formulation of this numerical criterion in the case
of Mumford-Tate varieties X. In this way, we give necessary and sufficient
criteria for a subvariety Y of X to be a special subvariety in the sense of the
Andre-Oort conjecture. We discuss in detail the important case where X=A_g, the
moduli space of principally polarized abelian varieties.
