We established shhact/+fish carrying the transgene Tg(ubb:iCRE-GFP), in which the expression of codon-improved Cre (iCRE) DNA is expressed by the ubiquitin B (ubb) promoter (Figure 2—figure supplement 1A). Although the ubb:Cre-GFP; shhact/+line is not the non-mosaic line that the reviewer suggested we establish, we suspect that the gene trap allele is inverted in all cells, including germ cells, of this line due to the strong and ubiquitous activity of the ubbpromoter. Consistent with global inversion in this strain, PCR genotyping of 48 single embryos prepared from incrosses of this line showed that all embryos contained the wild-type and/or the inverted mutagenic allele, and no embryos maintained the non-mutagenic allele, irrespective of the presence of the Cre transgene (Figure 3—figure supplement 2A and B). Thus, mosaicism with the mutagenic allele was virtually undetectable in the offspring of the ubb:Cre-GFP; shhact/+line.