Using stage-specific markers to identify cells associated with spontaneous remyelination, and to obtain purified population of oligodendrocyte lineage cells for transplantation into experimental models of demyelination, the long-standing debate about the origin of remyelinating oligodendrocytes is being resolved in favour of the adult oligodendrocyte progenitor (OP) (Groves et al., 1993; Warrington et al., 1993; Keirstead and Blakemore, 1997; Car-roll et al., 1998; Crang et al., 1998; Redwine and Armstrong, 1998; Levine and Reynolds, 1999) .