FISH with 5p subtelomeric probes and 18p subtelomeric probe further confirmed that the derivative chromosome 5 has derived from a translocation between 5p and 18p, which has given rise to a 46,XY,der(5)t(5;18)(p15.1;p11.31)dn karyotype.A de novo 5p partial deletion in conjunction with a cryptic 18p duplication has been detected in a boy featuring Cri-du-Chat syndrome.