Common shrews have an XX/XY1Y2 sex chromosome system, with the "X" chromosome being a translocation (tandem fusion) between the "original" X and an autosome; in males this autosome is represented by the Y2 chromosome. From G-banded chromosomes, the Y2 is homologous to the long arm and centromeric part of the short arm of the X. The region of the X that is homologous to the Y2 and also the telomeric region of the short arm of the X were found to be early replicating in somatic cells from a female shrew after 5-bromo-2'-deoxyuridine (BrdU) treatment in vitro. The remainder of the short arm of the X was shown to be late replicating. Electron microscopic examination of synaptonemal complexes in males at pachytene revealed pairing of the Y2 axis with the long arm of the X, and Y1 with the short arm. At early stages of pachytene, there is apparently extensive nonhomologous pairing between the X and Y1. In essence, the short arm of the shrew X chromosome behaves like a typical eutherian X chromosome (it is inactivated in female somatic cells and is paried with the Y1 during male meiosis) while the long arm behaves like an autosome (escapes the inactivation and pairs with the Y2).