Abstract 826

The 18q- syndrome, one of the most common chromosomal deletion syndromes, is caused by loss of genetic material from the long arm of chromosome 18. Most individuals with this syndrome exhibit mental retardation (93%), yet previous attempts to link cognitive status to deletion size have shown no association (Mahr et al., 1996; Strathdee et al., 1995). We studied 27 children ages 3 to 18 years of age. Deletion ranging from a proximal breakpoint at 18q21.1 to a more distal breakpoint at 18q23 were characterized using PCR-based polymorphic markers and molecular cytogenetic techniques (Cody et al., 1997). Upon detailed molecular analysis, we excluded 7 children because of more complicated reaarangements (5) and mosaicism (2). Intelligence, as measured by the age appropriate Wechster Scale. ranged from <20 to 97. Molecular ordering of the deletion breakpoint was correlated with IQ scores for the 20 children (ages 3 to 13 years) using Spearman Rank Order Correlation, which was significant at R = .67, p < .01. The detailed molecular analysis and inclusion of only those children with a "pure" deletion of genetic material may have enabled the association of cognitive ability with deletion size to emerge.