Many disease-causing mutations affecting donor splice site recognition are reported in the literature. One of the more frequently observed nucleotide changes causing aberrant splicing are due to mutations in the donor splice site which lower the strength of base pairing with U1 snRNA (small nuclear RNA). However, recent data have highlighted the possibility of a recognition mechanism for weak donor splice sites that are at least partially U1-independent. This is important as most of the donor splice site prediction programs currently in use are based on the U1 snRNA 5'-splice site base pairing and would not pick this up. We review these mechanisms and how an up-to-date donor splice site mutation repertoire indicates the heterogeneity of the molecular mechanism. We suggest that, in clinical molecular genetics, it is important to evaluate sequence variants for aberrant splicing even in those cases where the variant is not thought to alter the U1 snRNA interaction.