12Aberrant splicing is a major cause of rare diseases, yet its prediction from genome 13 sequence remains in most cases inconclusive. Recently, RNA sequencing has proven to 14 be an effective complementary avenue to detect aberrant splicing. Here, we developed 15 FRASER, an algorithm to detect aberrant splicing from RNA sequencing data. Unlike 16 existing methods, FRASER captures not only alternative splicing but also intron retention 17 events. This typically doubles the number of detected aberrant events and identified a 18 pathogenic intron retention in MCOLN1. FRASER automatically controls for latent 19 confounders, which are widespread and substantially affect sensitivity. Moreover, 20 FRASER is based on a count distribution and multiple testing correction, reducing the 21 number of calls by two orders of magnitude over commonly applied z score cutoffs, with 1 a minor sensitivity loss. The application to rare disease diagnostics is demonstrated by 2 reprioritizing a pathogenic aberrant exon truncation in TAZ from a published dataset. 3 FRASER is easy to use and freely available. 4