3 M tetramethylammonium chloride buffer (TMAC) is a hybridization buffer that selectively raises the stability of A:T base pairs to approximately that of G:C base pairs (11) (12) (13) (14) . It was used in these studies to reduce the effect of sequence composition when comparing different probes of the same length. The term nucleation site is used in this article to indicate a stretch of contiguous perfectly matching nt, capable of initiating hybridization (15) .