Second, they indicate that P. falciparum parasites regularly and spontaneously switch to expression of VSA PAM in the absence of an external signal, for example pregnancyassociated hormonal changes. It has been speculated that switching to VSA PAM expression requires signals from the pregnant host, for example hormones, and that selection therefore might not be possible unless the parasite is derived from such a host. It has also been argued that selection of parasites by panning on CSA in vitro might result in expression of antigens of dubious relevance to the antigens expressed as a result of in vivo selection occurring in the pregnant woman. Our data show that switching to expression of genuine VSA PAM can occur in vitro in the absence of external signals, in line with other recent evidence (18) . By extension, these findings suggest that switching to VSA PAM in vivo also occurs spontaneously regardless of the pregnancy status of the host. In a pregnant host, such parasites will often be at a selective advantage (because of the frequent absence of VSA PAM -specific immunity in women of low parity) (19) , whereas they appear to be unable to survive in a nonpregnant host (20) .