A gene is the basic unit of heredity in a living organism.More helpful is probably the definition recently proposed by Graziano Pesole:
A gene is a discrete genomic region whose transcription is regulated by one or more promoters and distal regulatory elements and which contains the information for the synthesis of functional proteins or non-coding RNAs, related by the sharing of a portion of genetic information at the level of the ultimate products (proteins or RNAs)The take-home lesson is that a genetically-encoded assay should not necessarily rely on expressed proteins.
PESOLE, G. (2008). What is a gene? An updated operational definition Gene, 417 (1-2), 1-4 DOI: 10.1016/j.gene.2008.03.010