During the cyst formation of Trichinella spiralis, the infected muscle cell undergoes basophilic change and apoptosis, which results in nurse cell formation. This study revealed expression kinetics of some apoptosis genes such as p53 and its closely related genes (tumor suppressor genes p53, p53; mouse double minute 2, MDM2; cyclin-dependent kinase inhibitor p21, p21waf). RT-PCR (reverse transcription polymerase chain reaction) results showed that these genes were temporarily expressed in the infected muscles during the cyst formation period, but not in normal muscles (or very low if any), which suggested the involvement of these apoptosis genes in the nurse cell formation. Cysts and neighbouring muscle cells were separately collected and RT-PCR was performed, which suggested that p53 was expressed in the cysts. An immunocytochemical study showed that p53 was expressed in the nucleoplasm of basophilic cell in the cyst and Trichinella larvae, which suggested involvement of these apoptosis genes in the nurse cell formation. The same p53 expression kinetic study was performed on p53 knockout mice. The knockout mice did not express p53 genes, but expressed the other apoptosis genes in the same kinetics with only minor exceptions, suggesting that the expressions of these genes during the cyst formation were more or less p53-independent. There were no differences in the number and morphology of the cysts between the knockout mice and wild type mice. Thus apoptosis seen during the Trichinella cyst formation can be operated in the presence or absence of p53.