A conditional protein tag sequence has been inserted at the LRP1 locus, replacing the endogenous stop codon without removing any coding or regulatory sequences. The inserted sequence comprises 1. an FRT cassette that contains a 1xTag:V5 tag followed by a stop codon and a single loxP site, 2. a 3xTag:FLAG tag followed by a stop codon. In the absence of FLP, the LRP1 protein is tagged with Tag:V5. If the FRT cassette is excised using FLP, the LRP1 protein is instead tagged with Tag:FLAG.