Mediates the voltage-dependent sodium ion permeability of excitable membranes. Assuming opened or closed conformations in response to the voltage difference across the membrane, the protein forms a sodium-selective channel through which Na(+) ions may pass in accordance with their electrochemical gradient. Further isoforms have been identified but not yet sequenced. These have different combinations of the optional exons A, B, C, D, E and F. Isoforms always have either exon C or D as these encode segment S4. Sequence identity to para from D.virilis suggests there may also be optional exons H and I. Isoform exonb and isoform D are seen in embryos and adults. Isoform A, isoform F24 and isoform F30 are predominant in embryos and isoform C and isoform E predominant in adults. The sequence contains 4 internal repeats, each with 5 hydrophobic segments (S1, S2, S3, S5, S6) and one positively charged segment (S4). Segments S4 are probably the voltage-sensors and are characterized by a series of positively charged amino acids at every third position. Partially edited. Further sites are edited by Adar. Positions 1455 and 1587 show minimal editing from embryos through to third larval instar, then a 40-fold increase at pupation. Position 471 has slightly higher levels during early development with only a four-fold increase at pupation. Belongs to the sodium channel (TC 1.A.1.10) family. Para subfamily.