Other Clone Isolation Strategies

Other strategies were also used to isolate clones that correspond to the unique subtelomere regions. Subtelomeric sequence-mediated walking was employed to identify clones for some human telomeres, such as 22q (Ning et al., 1996). A probe for TelBam3.4, a subtelomeric repeat sequence present on chromosome 22q as well as other human telomeres, was used to screen a total human P1 library by hybridization.  Although initial clones hybridized to multiple telomeres in addition to 22q by FISH, chromosome walking out of the telomeric repeats enabled the identification of a unique clone for the 22q telomere. Remaining clones for the unique subtelomere regions were identified by screening genomic libraries with primers designed from complete sequence information (Rosenberg et al., 1997).

Each unique human subtelomere clone is estimated to be within 300 kb of the end of each chromosome either via the inclusion of a sequence tagged site (STS) for a half-YAC vector-insert junction, an association with subtelomeric repeats, or complete sequence information. The telomeric location and chromosome specificity of each subtelomere clone was verified by FISH. The probes were also tested in a subset of individuals to rule out any common polymorphisms.