Both normal and pathological transcripts of tissue-specific genes may be detected by polymerase chain reaction (PCR) amplification in tissues not normally considered to express the gene product. The exploitation of constitutive basal mRNA levels ("ectopic" transcription) would be a major boon to diagnostic medicine since it promises both to simplify the analysis of complex genes and to avoid the requirement for an expressing tissue that is sometimes obtainable only by biopsy. We have demonstrated the feasibility of this novel strategy by characterizing a mutation in the X-chromosomal Duchenne (or Becker) muscular dystrophy (DMD/BMD) gene encoding dystrophin. The massive size of this gene has in the past often hindered carrier detection due to the high frequency of recombination and the high proportion of new mutations. In this study a deletion was identified in both a BMD patient and a heterozygous carrier using only a minimal volume of peripheral blood. Following specifically primed reverse transcription of lymphocyte RNA, the relevant region of the pathological cDNA was PCR-amplified. Sequence analysis indicated an in-frame deletion of exons 45 to 47.