We observed an apparent series of insertions and deletions beginning 5 bp downstream of an A-->G silent transition in exon 1 of the tumor necrosis factor receptor 1 gene. The apparent sequence anomaly was observed only in individuals carrying the transition. Formamide gel electrophoresis revealed that the apparent sequence anomaly was due to compression. The compression is plausibly explained by a hairpin in the reaction products in a region of trinucleotide CAG repeats. One should suspect the presence of DNA compression when a series of deletions and insertions follows a single base pair mutation that leads to a series of trinucleotide repeats.