A three-stage lean burn combustion chamber comprises a primary combustion zone, a secondary combustion zone and a tertiary combustion zone. The cloth is preferably stretched taunt over the frame for minimal movement in response to speaker excursion. The orthodontic jig can be connected with a cusp tip reference means orand a lingual bracket positioning device. The oxygen mask can be housed in a compartment inside the sitting surface of the seat. The invention provides an easy and powerful screening method in eukaryotic cells, such as insect cells, plant cells or mammalian cells, for ligands of orphan receptors, preferably of the multimerizing receptor type, for unknown ligands of known receptors, preferably multimerizing receptors, and for the genes encoding these ligands.