TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Cell: At least one of the transport proteins of the cell has an amino acid sequence the same as RasD or RasE. The cell preferably has transport proteins with the same amino acid sequence as RasD and RasE, and where the secretion signal does not comprise the Ras protein sequence given in the specification.Preferred Method: The oligonucleotide is labeled and the determination is by detection of the presence of label bound to the DNA. The determining is by amplification of DNA with the oligonucleotide as a primer, followed by detection of an amplified product.Preferred Construct: The construct further comprises an operably linked promoter recognized by Caulobacter. The secretion signal has an amino acid sequence which does not comprise the Ras sequence.Preferred Cell: The cell is a Caulobacter, preferably a C. crescentus cell. The DNA construct further comprises an operably linked promoter recognized by Caulobacter and where the DNA construct is expressed in the cell and the protein so expressed is secreted by the cell.Preparation: The cell may be prepared by standard methodologies. EXAMPLE - Fusion of a carrier cassette containing heterologous DNA segments to a C-terminal S-layer protein segment plasmid resulted in a plasmid that is not maintained in Caulobacter. Selection for the antibiotic marker on the plasmid resulted in detection of the rescue events. Most commonly these were single crossover homologous recombination events. The result was a direct insertion of the entire plasmid into the chromosome. Therefore the resident copy of the S-layer gene remained unchanged as well as the incoming modified S-layer gene. In such cases it was desirable to use Caulobacter strains in which the resident S-layer gene has been inactivated by adapting known procedures. One example was C. crescentus strain CB15AKSac which had an antibiotic resistance gene cassette introduced at a position in the S-layer gene about 25% of the way from the 5' terminus.