LACK OF X-INACTIVATION - LOSS OF ONE X-INACTIVATION CENTER IN A CASE WITH 45,X,-21,-21)(P21.3-P11.2)(DER(21)T(X)46,X,T(X-21) (P21.3-P11.2)/

We present a girl with a mos45,X, - 21, + der (21)t(X;21) (p21.3;p11.2 )/46,X,t(X;21) (p21.3;p11.2) chromosome constitution. The ratio of the se cells was 59/26 in phytohemagglutinin (PHA)-stimulated lymphocytes. The 45,X,der(21) t(Xp -;21p +) cells lacked an X inactivation center located at Xq13 on the derivative X chromosome; in these cells, the wh ole normal X chromosome and the distal part of Xp translocated onto th e derivative chromosome 21 were early replicating. She had moderate me ntal retardation and other findings different from those that occur in the Ullrich-Turner syndrome. Her phenotype may be due to the function al excess of the distal part of Xp on the derivative 21 in 45,X,der(21 )t(Xp -;21p +) cells; thus, this might be another type of the ''lack o f X-inactivation'' syndrome. (C) 1993 Wiley-Liss, Inc.