A combined system comprising two separate components, wherein the first component is a tag-exchange donor vector (TEDV) encoding a first cell surface tag exon flanked by a 3' intron fragment, and a gene of interest in the antisense orientation, the TEDV containing no promoter sequences, and the second component is an engineered cell containing within its genome a tag-exchange receiver site (TERS), encoding a second CST exon, which is different to the first CST, adjoined by a full intron sequence to an exon encoding a transmembrane domain, and also encoding a reporter gene in the antisense orientation, wherein paired recombinase mediated cassette exchange (RMCE) elements are included in the TEDV and TERS such that execution of RMCE between the TEDV and TERS results in exchange of the reporter element for the GOI encoded by the TEDV, and exchange of the first CST exon for the second CST exon, such that the derivative engineered cell now expresses the first CST and GOI, in place of the second CST and the reporter gene, wherein the first component is a TEDV comprising from 5' to 3' a. a first 5' RMCE element encoded in the non-coding and non-functional 3' intron fragment b. a 3' intron fragment c. an exon comprising TEDV-encoded CST in the 5' to 3' direction d. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction e. a second transcriptional terminator sequence for the 3' to 5' encoded GOI f. a sequence encoding the GOI in the 3' to 5' direction g. a Kozak sequence h. a second 3' RMCE element wherein the CST exon and first transcriptional terminator are encoded in the antisense orientation from the GOI and from the associated transcriptional terminator and Kozak sequences, and wherein the second component is a TERS comprising from 5' to 3' a. a transcriptional promoter element b. a Kozak sequence c. an exon encoding a protein transmembrane domain d. a 5' intron e. a first 5' RMCE element encoded within the intron sequence d and paired with 5' RMCE element of TEDV. f. an exon comprising TERS-encoded CST in the 5' to 3' direction g. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction h. a second transcriptional terminator sequence for the 3' to 5' encoded selection gene i. a sequence encoding a reporter gene in the 3' to 5' direction j. a Kozak sequence k. a second 3' RMCE element paired with the 3' RMCE element of TEDV l. a second transcriptional promoter element wherein the transmembrane domain exon and CST exon are encoded in the antisense orientation from the reporter gene, such that the first transcriptional promoter element drives transcription of the combined transmembrane domain and CST, and the second transcriptional promoter element drives transcription of the reporter gene. wherein the first component is a TEDV comprising from 5' to 3' a. a first 5' RMCE element encoded in the non-coding and non-functional 3' intron fragment b. a 3' intron fragment c. an exon comprising TEDV-encoded CST in the 5' to 3' direction d. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction e. a second transcriptional terminator sequence for the 3' to 5' encoded GOI f. a sequence encoding the GOI in the 3' to 5' direction g. a Kozak sequence h. a second 3' RMCE element a. a first 5' RMCE element encoded in the non-coding and non-functional 3' intron fragment b. a 3' intron fragment c. an exon comprising TEDV-encoded CST in the 5' to 3' direction d. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction e. a second transcriptional terminator sequence for the 3' to 5' encoded GOI f. a sequence encoding the GOI in the 3' to 5' direction g. a Kozak sequence h. a second 3' RMCE element wherein the CST exon and first transcriptional terminator are encoded in the antisense orientation from the GOI and from the associated transcriptional terminator and Kozak sequences, and wherein the second component is a TERS comprising from 5' to 3' a. a transcriptional promoter element b. a Kozak sequence c. an exon encoding a protein transmembrane domain d. a 5' intron e. a first 5' RMCE element encoded within the intron sequence d and paired with 5' RMCE element of TEDV. f. an exon comprising TERS-encoded CST in the 5' to 3' direction g. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction h. a second transcriptional terminator sequence for the 3' to 5' encoded selection gene i. a sequence encoding a reporter gene in the 3' to 5' direction j. a Kozak sequence k. a second 3' RMCE element paired with the 3' RMCE element of TEDV l. a second transcriptional promoter element a. a transcriptional promoter element b. a Kozak sequence c. an exon encoding a protein transmembrane domain d. a 5' intron e. a first 5' RMCE element encoded within the intron sequence d and paired with 5' RMCE element of TEDV. f. an exon comprising TERS-encoded CST in the 5' to 3' direction g. a first transcriptional terminator sequence for encoded CST in the 5' to 3' direction h. a second transcriptional terminator sequence for the 3' to 5' encoded selection gene i. a sequence encoding a reporter gene in the 3' to 5' direction j. a Kozak sequence k. a second 3' RMCE element paired with the 3' RMCE element of TEDV l. a second transcriptional promoter element wherein the transmembrane domain exon and CST exon are encoded in the antisense orientation from the reporter gene, such that the first transcriptional promoter element drives transcription of the combined transmembrane domain and CST, and the second transcriptional promoter element drives transcription of the reporter gene.