once the gene that is disrupted in the "bicoid" mutant was identified, it was shown that "bicoid" mrna is transcribed in the nurse cells and then relocalized to the oocyte. other maternal effect mutants either affect products that are similarly produced in the nurse cells and act in the oocyte, or parts of the transportation machinery that are required for this relocalization. since these genes are expressed in the (maternal) nurse cells and not in the oocyte or fertilised embryo, the maternal genotype determines whether they can function.