Fig. 2. In situ hybridization for XNr1 in untreated and concanamycin-treated Xenopus embryos. Treatment of Xenopus embryos with the H⁺-V-ATPase inhibitor concanamycin causes significant levels of heterotaxia that can be seen in the situs of organs at stage 45 (Fig. 1L), but disruptions of laterality can be detected much earlier by the expression patterns of normally left-sided markers. (A-D) Sectioned embryos processed for in situ hybridization with an XNr1 antisense probe. In wild-type embryos, XNr1 is restricted to the left (A); however, in inhibitor-treated embryos, its expression is randomized (B-D). Green arrows indicate normal position of staining; red arrows indicate ectopic expression domain; white arrows indicate lack of expression in normal region. Dorsal is upwards in all panels; left and right sides correspond to those of the reader.