Splicing is an essential cellular process to generate mature transcripts from pre-mRNA. It requires the splice factor U1 small nuclear ribonucleoprotein (U1), which promotes exon recognition by base-pairing interaction with the splice donor site (SD). After U1 dissociation, exon recognition is maintained by U6 small nuclear ribonucleoproteins (U6). It has been shown that SD mutations lower the binding affinity of U1 and cause splice defects in about 10% of patients with monogenetic diseases. U1 isoforms specifically designed to bind the mutated SD with increased affinity can correct these splice defects. We investigated the applicability of this gene therapeutic approach for different mutated SD positions. A minigene-based splicing assay was established to study a typical SD derived from the gene BBS1. We found that mutations at seven SD positions caused splice defects. In four cases, mutation-adapted U1 isoforms completely corrected these splice defects. Partial correction was found for splice defects induced by the mutation at SD position +5. The limited therapeutic efficacy at this position was alleviated by applying a combined treatment with mutation-adapted U1 and U6. The sequence complementarity between U6 and three SD positions (+4, +5,and +6) was relevant for the outcome of the therapy. Between 30 and 100% of the normal transcripts can be restored. The treatment significantly decreased both exon skipping and intron retention. Massive missplicing of off-target transcripts was not detected. Our study helps to assess the therapeutic efficacy of mutation-adapted U snRNAs in gene therapy and illustrates their strong potential to correct splice defects, which cause many different inherited conditions.