A tetranucleotide repeat located in intron 20 of the RB gene consists of 16-26 CTTT(+/- T) repeats in 99% of the alleles. In the remaining 1% of alleles the segment is extended to a length of > 60 repeats. Sequence analysis revealed that the prolonged alleles either consisted of perfect CTTT(+/- T) repeats or irregular repeat sequences with variable combinations of C and T. The data provide clues to the mechanisms causing unstable expansions of repeats.