Screening assays and methods

A method of producing a microarray of cell-derived products comprising:

• (a) reversibly sealing a moldable slab to a surface of a substrate, said moldable slab comprising an array of microwells, each microwell having a side or diameter of less than 100 microns and containing not more than a single or a few cell(s), at least one of said microwells containing the single or few cell(s), wherein the single or few cell(s) produce a cell-derived product in a volume of 10 nanoliters or less of fluid in each said microwell containing the single or a few cell(s);
• wherein the surface of the substrate facing each said microwell is modified such that it acts to retain the cell-derived product on the surface of the substrate;
• and
• (b) allowing said single or few cell(s) to produce said cell-derived product in said volume such that the cell-derived product produced by the single or few cell(s) in said volume is retained in a detectable amount on the surface of the substrate sealed to the moldable slab.