Frequency of 13q abnormalities among 203 patients with retinoblastoma

We studied peripheral blood lymphocyte karyotypes of 203 patients with retinoblastoma. Twelve (5.9%) had a constitutional chromosomal abnormality involving 13q, of whom six had unilateral and six had bilateral disease. Two patients had mosaic deletions, eight had nonmosaic deletions, one had a de novo translocation, and one had a 13q14 deletion and a de novo direct insertion (10;6). Of the total, 4.9% of unilateral and 7.5% of bilateral patients had 13q abnormalities. None of 19 familial retinoblastoma patients had a visible cytogenetic abnormality. The unilateral patients with 13q abnormalities represent prezygotically determined (potentially heritable) cases which would have been classified as postzygotic (sporadic) without cytogenetic analysis. The observed 1% frequency of mosaic deletions is lower than that previously reported.