Repetitive sequence tracts, including homopolymer, homopurine and homopyrimidine, are highly represented in Table 2 . Upon closer inspection, it became apparent why the cross-hybridization is strong for probes in homopolymeric regions. In these sequence contexts, substitution and deletion probes can form duplexes with wild-type target that are longer than 12 bp in length. For example, the probe designed to detect a single base deletion at position 633 is designed to form one 12 bp and one 13 bp duplex with wild-type target. However, this probe can form duplexes that range from 12 to 18 bp in length with wild-type sense strand target due to slippage ( Figure 5 ). This type of ambiguity leads to increased stability of these DNA-RNA heteroduplexes (34) .