An osk fragment containing the 15bp 5' untranslated region (UTR), the first in-frame start codon (m1) and 414 nucleotides downstream of m1 (this fragment lacks the second in-frame start codon - m2) is fused to a 3kb lacZ fragment. A 1040bp wild-type genomic osk fragment, beginning with the stop codon and containing the 3' UTR, is fused downstream of lacZ.