Two out of 40 clones already showed an inverted I-XCI profile, 25 clones displayed low levels of Xist expression overall combined with a bi-allelic expression of both Cox7b and Pgk1a, while 13 clones exhibited a preferential paternal expression of Xist associated with strong bi-allelic expression of Cox7b and Pgk1. Since this latter expression profile evolves, after HAT release (passage 8), towards a return to a parental-like XCI pattern, these clones have probably locally and transiently reactivated both XP-linked loci together with the Hprt1WT gene. This evidence concerns the gene COX7B and xeroderma pigmentosum.