Expresses an osk transgene from a genomic fragment that includes the 5' extended UTR. The start codon for translation of isoform long-osk, M1, is mutated (as in osk^M1R). Because long-osk is not produced by the transgene, mutations that lie between the start codons for long-osk and short-osk will only alter the mRNA sequence, and not the protein coding sequence. The transgene includes a 3xHA epitope tag, inserted after amino acid residue T140. The short-osk start codon is M139. Carries deletion 361-410 in the extended 5' UTR of osk.