NxtIRFdata is a companion package for SpliceWiz, an interactive analysis and visualization tool for alternative splicing quantitation (including intron retention) for RNA-seq BAM files. NxtIRFdata contains Mappability files required for the generation of human and mouse references. NxtIRFdata also contains a synthetic genome reference and example BAM files used to demonstrate SpliceWiz's functionality. BAM files are based on 6 samples from the Leucegene dataset provided by NCBI Gene Expression Omnibus under accession number GSE67039.