The use of a tag moiety comprising a biotinylation domain, such as biotin carboxyl carrier protein (BCCP), as a protein folding marker and protein solubility enhancer in the orientated surface capture of products of heterologously expressed genes is described. Methods for increasing the solubility of proteins and determining the folded state of a protein are also disclosed. The uses and methods of the invention can be carried out in a multiplexed manner on more than one protein in the formation of libraries. In addition the nucleic acid molecule encoding the biotinylation domain of the tag moiety can be used to increase the proportion of clones in a library that express the protein of interest.