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Fig. 7 Notch is required for the induction of AV conduction tissue. (A) notch1b knockdown embryos fail to develop an AV conduction delay, as shown in this calcium activation map of a single cardiac cycle in a notch1b morphant at 48 hpf. Isochronal lines (50 mseconds) obtained by fluorescence microscopy are superimposed on a maximum intensity projection image. (B) notch1b morphants fail to develop an AV conduction block on treatment with terfenadine, as seen in this histogram of the proportion of embryos developing 2:1 AV block at 48 hpf. Error bars indicate s.e.m. (C) neuregulin morphants exhibit normal expression of notch1b, as seen in this in situ hybridization. Arrowhead indicates cardiac expression in the AV ring. (D) notch morphants exhibit normal expression of neuregulin, as seen in this in situ hybridization. Arrowhead indicates cardiac expression in the AV ring.