13q-/r(13) mosaicism.

A 2-month-old female infant with typical features of the 13q-syndrome was found to be a hitherto unreported mosaic consisting of 46,XX,del(13)(q22)-46,XX,r(13)(p13q22). Both of the 13q- and r(13) chromosomes were Ag N banding positive. Therefore, it was assumed that they had retained the satellite stalks. Two possible mechanisms were proposed for the genesis of the mosaicism. Firstly, the patients started with the 13q- chromosome, which then underwent breakage and reunion at both ends to form the r(13) chromosome. Secondly, the patients started with the r(13) chromosome, which reopened at or close to the joining point to form the 13q- chromosome.