**4. Discussion**

Molecular genetic studies have shown that hemizygosity of the elastin (*ELN*) gene accounts for the cardiovascular abnormalities observed in WBS patients. The other signs (facial dysmorphism, growth and mental retardation, etc.) observed in WBS are likely to be due to hemizygosity of other genes flanking the *ELN* locus. Although the unique combination of signs and anomalies is highly evocative, WBS patients usually present phenotypic variability, which could be explained by different deletion size and, therefore, by different sets of genes showing hemizygosity.

Several studies tried to define the WBS critical deletion region. Using sequence tagged site (STS) markers flanking the *ELN* gene, Perez-Jurado et al. (1996) and Wu et al. (1998) looked for the minimal deletion region responsible for the WBS phenotype in 123 patients. Markers from D7S489B (also named D7S489U) to D7S1870 were shown to be consistently removed, defining a 2 cM deletion in all informative patients (Perez Jurado et al., 1996; Wu et al., 1998).

Using BAC and PAC (P1-derived artificial chromosome) clones, Meng et al. (1998) constructed a physical map of the common deletion region. They localized the centromeric and telomeric deletion breakpoints to two genomic clones (containing D7S489B and D7S1870) flanking a 1.5 Mb deletion interval (Meng et al., 1998).

Based on the work by Valero et al. (2000) who found that 3 large region-specific segmental duplication or low copy repeat (LCR) elements flanked the common deletion region (Valero et al., 2000), Bayes et al. (2003) defined two common deletion regions in a set of 74 patients with WBS. Most of the patients (95%) had a 1.55 Mb deletion whereas the remaining 5% exhibited a larger deletion of approximately 1.84 Mb (Bayes et al., 2003).

Botta et al. (1999) reported a deletion of about 850 kb in two patients showing the full spectrum of the WBS phenotype (Botta et al., 1999). Furthermore, the analysis of the deletion size in WBS patients with "incomplete" phenotype revealed even more heterogeneity (Antonell et al., 2010). Using quantitative real-time PCR to scan 2.5 Mb of the WBS deletion region at a resolution of 100-300 kb among 65 patients with strong clinical indication of having WBS, Schubert and Laccone (2006) found that 21 patients had a deletion in the WBS region. Nineteen patients had a deletion ranging from 1.4 to 1.8 Mb in size whereas one had a 200 kb deletion and the remaining one a 2.5 Mb deletion (Schubert & Laccone, 2006).

FISH using BAC clones has been extensively used to construct physical maps of the 7q region deleted in WBS and to define the commonly deleted region (Meng et al., 1998; Peoples et al., 2000; Perez Jurado et al., 1996; Wu et al., 1998). However, to our knowledge, no study has been conducted to determine the deletion size in WBS individuals using overlapping BACs. Indeed, the level of resolution is a limit to this technique. A BAC is considered as partially deleted by FISH analysis when the fluorescent signal of this clone is much stronger on one chromosome 7 than on the other. Furthermore, a BAC could be considered as present or absent (not partially deleted) if only a small part of the DNA sequence is removed or kept (usually less than 50 kb). Having resource to overlapping BAC clones can increase the level of confidence but some uncertainty will remain.
