Although dicots like Arabidopsis has an endosperm which is consumed during embryo development, the stages of endosperm development from the fertilized triploid endosperm through the endosperm coenocyte to the cellular endosperm is highly conserved between Arabidopsis and the cereals. The main control points in this process are (i) suppression of functional phragmoplast during the first round of mitotic divisions, (ii) mitotic arrest, (iii) formation of RMS (radial microtubular systems) and deposition of cell walls in RMS interzones, (iv) release of mitotic arrest and (v) determination of aleurone cell fate in the outer endosperm cell layer. We have a collection of 40 Arabidopsis promoter trap lines expressing the GUS marker gene in developing endosperm, aleurone, and/or embryo, isolated in our Strategic University Program ">From DNA sequence to gene function using Arabidopsis thaliana as a tool". The lines have been isolated from a collection of T-DNA lines transformed with a promoterless gus construct. We expect that the characterization of the pattern of expression as well as the function of these genes will contribute valuable information and t ools towards the long term strategy to understand the underlying controls in endosperm development, and elucidate the differences between monocot and dicot seed development.