The human sex chromosomes differ in sequence, except for the pseudoautosomal 
regions (PAR) at the terminus of the short and the long arms, denoted as PAR1 
and PAR2. The boundary between PAR1 and the unique X and Y sequences was 
established during the divergence of the great apes. During a copy number 
variation screen, we noted a paternally inherited chromosome X duplication in 15 
independent families. Subsequent genomic analysis demonstrated that an 
insertional translocation of X chromosomal sequence into the Y chromosome 
generates an extended PAR [corrected].The insertion is generated by non-allelic 
homologous recombination between a 548 bp LTR6B repeat within the Y chromosome 
PAR1 and a second LTR6B repeat located 105 kb from the PAR boundary on the X 
chromosome. The identification of the reciprocal deletion on the X chromosome in 
one family and the occurrence of the variant in different chromosome Y 
haplogroups demonstrate this is a recurrent genomic rearrangement in the human 
population. This finding represents a novel mechanism shaping sex chromosomal 
evolution.