Germline mutations in the BRCA1 gene are scattered over the 22 coding exons and most of them generate premature termination codons (PTCs). A mechanism called nonsense-mediated mRNA decay (NMD) is known to specifically degrade transcripts with PTCs; however, steady-state amounts of mutant BRCA1 mRNAs have very rarely been measured. Although growing evidence implicates downstream exon–exon junctions (EEJs) as critical determinants for discrimination between normal stop codons and PTCs, requirements concerning the minimal and maximal distance between PTCs and downstream EEJs are still debated. We assessed the relative amount of transcripts encoded by BRCA1 alleles harbouring 30 different truncating mutations in lymphoblastoid cell lines established from carriers from breast/ovarian cancer families. We found that NMD is triggered by 80% of PTC⁺ alleles and results in a 1.5- to 5-fold reduction in mRNA abundance. All truncating mutations located in the 3.4 kb long central exon are subject to NMD, irrespective of their distance to the downstream EEJ (305 to 3395 nt). PTCs not leading to NMD are either located in the last exon or very close to the translation initiation codon. We hypothesize that reinitiation could explain why transcripts carrying early PTCs escape NMD. This is the first study challenging the NMD rules, which have been established through the study of minigenes, by analysing a large series of mutant endogenous alleles.