Expression domains of selected marker genes in the Xenopus pronephros and the rodent metanephros. The expression domains of selected marker genes in the nephrons of (a) Xenopus stage 35/36 pronephric kidneys and (b) adult rodent metanephric kidneys are depicted schematically. The nephrostomes, which may correspond to the neck segment found in some mammalian species such as rabbits [67,68], are not shown. Dark and pale blue colors indicate strong and low expression levels, respectively. Note that only a single clcnk gene is known in Xenopus, whereas there are two mouse Clcnk genes. The expression of Xenopus clcnk in the intermediate, distal, and connecting tubules has therefore to be compared with the combined renal expression domains of mouse Clcnka and Clcnkb. The abbreviations for segments of the pronephric and metanephric nephrons are given in the legend to Figure 2.