We examined the peripheral blood chromosomes of eight patients with retinoblastoma. In two of them an interstitial deletion of 13q was found. The breakpoints were determined as follows: case 1, 13q12→21; case 2, 13q12→31. In both cases, band 13q14 was deleted. In case 2 the lymphocytes of the mother showed the identical interstitial 13q deletion in 3 of 100 mitoses, thus raising the possibility of maternal origin of the 13q deletion in a child. In one patient, retinoblastoma was unilateral; in the other, bilateral. Both patients were mentally retar
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