An unbalanced Y to X translocation due to a de novo mutation is described in a female with some clinical features of the Turner syndrome. Her karyotype is defined as 46,X,t(X;Y)(Xp11.2;Yq11). Hae III restriction analysis revealed an amount of male-specific DNA sequences in the normal male range. DNA replication analysis showed that in all cells studied the translocation X chromosome was late replicating and that the X segment of the translocation chromosome was later replicating while replication of the Y segment varied. A serologic test indicated a reduced titer of H-Y antigen, and biochemical studies for the enzyme steroid sulfatase revealed activity in the male range.