 Rhombomeres, which extend down the embryo from the hindbrain, contain clear boundaries and each express various Hox genes that are necessary for differentiation of structures within the body.More anterior regions of the brain were examined in search of other cell lineage restriction boundaries, and multiple potential boundaries continue to be studied (see Developmental Boundaries).The importance of these compartments as local signaling centers, areas which chemically influence surrounding tissue, was elucidated by first observing differential expression of Hox genes in various compartments and second by observing mutant D. melanogaster and corresponding phenotypic (physical) changes.