Table 1. Relative abundances of extension products by match or mismatch at the 3′-termini of primers.

Template		Primer A	Primer C	Primer T	Primer G
P53-A	Modified primer	0.7 ± 0.3	0.0 ± 0.2	100.0 ± 0.0	0.0 ± 0.1
	Unmodified primer	0.0 ± 0.1	43.5 ± 0.2	100.0 ± 0.0	7.9 ± 0.5
P53-C	Modified primer	0.2 ± 0.1	0.4 ± 0.2	3.5 ± 0.3	100.0 ± 0.0
	Unmodified primer	28.5 ± 0.4	1.0 ± 0.5	28.0 ± 0.2	100.0 ± 0.0
P53-T	Modified primer	100.0 ± 0.0	0.2 ± 0.1	1.4 ± 0.2	0.0 ± 0.3
	Unmodified primer	100.0 ± 0.0	47.2 ± 0.3	13.9 ± 0.4	54.8 ± 0.3
P53-G	Modified primer	0.0 ± 0.2	100.0 ± 0.0	4.4 ± 0.3	2.8 ± 0.3
	Unmodified primer	0.3 ± 0.2	100.0 ± 0.0	41.3 ± 0.5	2.5 ± 0.1

The switching by modified primers was almost perfect while undesirable strand extension reactions occurred with unmodified primers. The test samples were synthetic human P53 fragments each containing a different base, A, C, T or G, at the polymorphic site, with the four variants named P53-A, P53-C, P53-T and P53-G (n = 2), respectively. Modified means that a mismatched base was artificially introduced into the primer at the third position from the 3′-terminus; unmodified means that the primer did not include such a base. The data was normalized to the maximum intensity in a set of four experiments (primers A, C, T and G) as 100.