3.2. Multiplex PCR using truncated primers As a strategy to increase the sensitivity of multiplex PCR, a set of truncated primers were designed. Under the experimental conditions described by Matsunaga et al. (1999), the pork, beef, chicken and mutton DNA could be identified solely by multiplex PCR using the shortened primers, and the size of amplified fragments were as expected. Yet, when beef DNA was mixed with other template, the beef-specific band was not observed (Fig. 3). Subsequently, we adjusted the ratio of primers, but beef could not been detected in the mixture even when the concentration of B2 was higher than SIM2 (figure not shown). A temperature gradient PCR was performed to seek the appropriate annealing temperature. However, beef was still not detected in the mixture when the annealing temperature was lowered to 49 _C. Simultaneously, a number of non-specific amplifications were observed (figure not shown). So, the identical efficiency on different templates could not been optimized using these truncated primers. Moreover, according to the lightness of bands, the sensitivity of multiplex PCR was not markedly improved.