Examples of prominent biochemical pathways identified in mouse oocytes. (a) The protein citrullination pathway has recently been shown to be essential for early development, as targeted mutation of Padi6 renders females infertile . Note that Padi6 is the only gene of the peptidyl arginine deiminase family expressed in oocytes. (b) The inability of glucose utilization by mouse oocytes may be due to the lack of hexokinases required for the first step in glycolysis. Genes next to the corresponding reactions are shown in black (expressed in the oocytes) or in grey (not expressed).