Abstract: Body fluid identification by mRNA profiling may allow extraction of contextual ‘activity level’ information from forensic samples. Accordingly, a prototype multiplex digital gene expression method for forensic body fluid/tissue identification is provided, based upon solution hybridization of color-coded (e.g., NanoString®) probes. For example, a model for gene expression in a sample from a single body fluid is provided and extended to mixtures of body fluids. A calculation of maximum likelihood estimates of body fluid quantities in a sample is performed, and use of likelihood ratios to test for the presence of each body fluid in a sample is described. A process/algorithm is described and, unlike conventional algorithms for detecting tissues and cells, may allow for zero false-positive fluid identifications across a plurality of samples. Such a protocol may facilitate routine use of mRNA profiling in casework (e.g., forensic) laboratories that previously has not been as reliable.