E10/I10 tau mutations. RNA sequences for part of E10 (A) and the E10/I10 junction (B) are shown. Capital letters are E10, and lower case letters are I10. The hatched box below the E10 sequence spans the proposed purine-rich exon splicing enhancer. The hatched box above the sequence spans the proposed exon splicing silencing sequence. The normal tau E10/I10 junction sequence is presented as a potential stem–loop. The E10 + 16 mutation was used in splicing experiments with the 305N mutation to attempt to restore putative stem–loop base pairing. Likewise, an E10 + 12 substitution was used in double-mutant constructs to compensate for the E10 + 3 mutation. The predicted free energies in kcal/mol (1 kcal = 4.18 J) at 37°C are: −9.2, normal (stem–loop shown); −6.8, 305N; −7.8, 305N//E10 + 16; −4.5, E10 + 3; −7.1, E10 + 3/E10 + 12; −6.7, E10 + 14; −7.8, E10 + 16 using the GCG version of mfold (39).