  In this paper we explore a connection between descriptive set theory and
inner model theory. From descriptive set theory, we will take a countable,
definable set of reals, A. We will then show that A is equal to the reals of M,
where M is a canonical model from inner model theory. In technical terms, M is
a ''mouse''. Consequently, we say that A is a mouse set. For a concrete example
of the type of set A we are working with, let OD(n) be the set of reals which
are Sigma-n definable over the omega-first level of the model L(R), from an
ordinal parameter. In this paper we will show that for all n, OD(n) is a mouse
set. Our work extends some similar results due to D.A. Martin, J.R. Steel, and
H. Woodin. Several interesting questions in this area remain open.
