Splicing of exon 9a in FMR1 transcripts results in a truncated FMRP with altered subcellular distribution.

•We identified a new 46 bp microexon terming the exon 9a.•Inclusion of exon 9a introduces an in-frame termination codon.•Exon 9a-containing transcripts could produce a 34 kDa truncated protein.•Truncated protein displayed both nuclear and cytoplasmic localization.