In the present study we found the mouse crumbs ortholog to be up-regulated in the rd7 mutant retina by microarray, consistent with its higher expression level in cones than in rods [50]. Although we were unable to confirm this finding by in situ hybridization due to the weakness of the signal, it is possible that the up-regulation of crumbs in the retina is the cause of the lamination defects seen in the rd7 mutant. Overexpression of wild-type crumbs in Drosophila has been shown to cause polarity defects leading to waviness of epithelia and even to misalignment of nuclei within photoreceptors analogous to what is seen in the rd7 retina [47,51]. Future experiments will address this question by overexpressing full-length Crb1 in a wild-type background.