A novel Tth111I restriction fragment length polymorphism (RFLP) allows tracing of X-chromosome inactivation in the (Xid) heterozygote.

The X-linked immunodeficiency (Xid) in CBA/N mice serves as a model for the X-linked agammaglobulinemia (XLA) syndrome in man. X-chromosome inactivation in F1 heterozygotes derived from CBA/N (Xxid/Xxid) and B6.Pgk-1a (X+/Y) was investigated by monitoring the methylation status of the individual Pgk-1 alleles, Pgk-1b and Pgk-1a, respectively, using a novel Tth111I RFLP. Results indicate that in circulating B lymphocytes of female heterozygotes, only the X chromosomes carrying the normal alleles (X+) are active (nonrandom inactivation of the X chromosome), whereas in non-B cells both the X chromosomes (X+ and Xxid) are active (random inactivation of the X chromosome). These results were further confirmed by direct evaluation of transcription of the Btk gene, the gene mutated both in Xid and in XLA.