Gene Ontology information related to the biological role of genes is organized in a hierarchical manner that can be represented by a directed acyclic graph (DAG). Treemaps graphically represent hierarchical information via a two-dimensional rectangular map. They efficiently display large trees in limited screen space. Treemaps have been used to visualize the Gene Ontology by first transforming the DAG into a tree. However this transformation has several undesirable effects such as producing trees with a large number of nodes and scattering the rectangles associated with the duplicates of a node around the screen. In this paper we introduce the problem of visualizing a DAG as a treemap, we present two special cases, and we discuss complexity results.