A tissue-specific fragile site associated with the sex reversed (Sxr) mutation in the mouse.

We have observed a chromosomal marker which has both the appearance and behavior of a fragile site and is associated with the mouse sex reversed (Sxr) mutation. The observation of a chromosomal fragile site at this location is of interest since it is a region of enhanced meiotic recombination, Sxr being adjacent to the site of exchange between the X and Y chromosomes in the male. However it is an unusual fragile site in two respects: it is spontaneously expressed in relatively high frequency and this expression is tissue specific. We have observed the fragile site in extraembryonic tissues, preimplantation embryos and premeiotic germ cells, all of which share the property of being undermethylated by comparison with embryonic tissues.