Nanowell array formats are used to express individual proteins in small volume reaction vessels or nanowells (Figure 4). This format is sometimes preferred because it avoids the need to immobilize the target protein which might result in the potential loss of protein activity. The miniaturization of the array also conserves solution and precious compounds that might be used in screening assays. Moreover, the structural properties of individual wells help to prevent cross-contamination among chambers. In 2012 an improved NAPPA was published, which used a nanowell array to prevent diffusion. Here the DNA was immobilized in the well together with an anti-GST antibody. Then cell-free expression mix was added and the wells closed by a lid. The nascent proteins containing a GST-tag were bound to the well surface enabling a NAPPA-array with higher density and nearly no cross-contaminations.