 These differences in gene expression could be categorized into four categories (Arthur 2004). One of these categories involves the place of gene expression, where different populations of cells express a particular gene in different species. For instance, the gremlin gene in the duck hindlimb webbing protects these cells from cell death, enabling webbed feet (Laufer et al, 1997; Merino et al. 1999). A second category involves changes in the timing of gene expression, as in the continued expression of the fgf8 gene at the tip of the dolphin forelimbs, thus enabling the extension of its flippers (Richardson &amp; Oelschläger 2002). A third category of change involves alterations in the magnitude of gene expression, as in the differences in Bmp4 gene expression that determine the width of finch beaks (Abzhanov et al. 2004). A fourth category focuses on the alterations of the actual protein sequence of regulatory proteins, as in the changes of the Antennapedia gene in insects, which restrict insects from forming more than six limbs (Galant and Carroll 2002).