X/XY mosaicism with short Y

A case of 45, X/46, XY mosaicism is reported in an infant with ambiguous penitalia. The patient's Y long arm is shorter than the father's one and has no bright fluorescence nor intensively Giemsa‐stained distal part. The paternal Y chromosome is normal but the light fluorescent and the pale Giemsa stained part of the long arm is shorter than the patient's Y. A complex chromosomal rearrangement is then more probable than a simple deletion.