The K D value between N TAIL and XD, as measured by both fluorescence spectroscopy and surface plasmon resonance, is in the 100 nM range. Surprisingly, this value is considerably lower than that reported by Kingston et al. (2004b) (13 mM) and derived from isothermal titration calorimetry studies. A weak binding affinity, associated with a fast association rate, would ideally fulfill the requirements of a polymerase complex, which has to cartwheel on the nucleocapsid template during both transcription and replication. However, a K D in the mM range would not seem to be physiologi-cally relevant considering the low intracellular concentrations of P in the early phases of infection. Moreover, such a weak affinity is not consistent with the ability to readily purify nucleocapsid-P complexes using rather stringent techniques such as CsCl isopycnic density centrifugation (Robbins and Bussell 1979; Stallcup et al. 1979; Robbins et al. 1980; Oglesbee et al. 1989) . Our data support a higher affinity between P and N, resulting in a stable P-N TAIL complex that would be predicted to hinder processive movement of P along the nucleocapsid template. In agreement with this prediction, Box3 has been shown to have an inhibitory role upon transcription and replication, as indicated by previous minireplicon experiments, where deletion of Box3 enhanced basal reporter gene expression (Zhang et al. 2002) . We can speculate that the transient nature of the N TAIL -XD interaction might be ensured by the possible intervention of cellular and/or viral cofactors. Indeed, the requirement for cellular or viral cofactors in both transcription and replication has been already documented in the case of measles (Vincent et al. 2002) , respiratory syncytial (Fearns and Collins 1999) , and Ebola viruses (Hartlieb et al. 2003) . These cofactors may serve as processivity or transcription elongation factors and could act by modulating the strength of the interaction between the polymerase complex and the nucleocapsid template. Such a mechanism may explain the stimulatory effect of hsp72 on MV transcription and genome replication, an effect mediated by Box3-hsp72 interaction (Zhang 2002) . In this capacity, hsp72 may neutralize the contribution of Box3 to a more stable complex between P and N TAIL , thereby promoting successive cycles of P binding and release that are essential to polymerase processivity.