DL encodes a protein containing two distinct domains: a zinc-finger domain in the N-terminal region and a C-terminal YABBY domain, which may form a helix-loop-helix structure and shares low similarity with the HMG box transcription factors (Yamaguchi et al., 2004). OsDL regulates midrib formation by promoting cell proliferation specifically along the adaxial-abaxial axis of leaves. In loss of function mutants, this specific cell proliferation does not to occur because the number of cell files in the center of the leaf is reduced and no obvious thickening of the central region is observed (Yamaguchi et al., 2004). In overexpression plants, the lateral regions of the leaf also became thick from vigorous cell proliferation, resulting in the formation of midrib-like structures in these regions (Yamaguchi et al., 2004).