A, B. Whole cell sodium currents (A), and current-voltage relationships in CHO cells transfected with full-length scn5Laa and scn5Lab (B). C. Temporal expression of scn5Laa and scn5Lab in early embryos (0–12hpf) by RT-PCR. YPC = yoke plug closure stage, s = somites. D, E. By whole-mount in situ hybridization, sodium channel gene expression was detected diffusely in both heart chambers before displaying predominately ventricular expression by 104hpf. Transcripts were also detected in the brain and spinal cord. Developmental stages are as labeled. Arrow, heart. Arrowhead, spinal cord. A, atrium. V, ventricle.