SINE-R elements constitute a class of retroposons derived from the long ter
minal repeat (LTR) of the human endogenous retrovirus HERV-K family that ar
e present in hominoid primates and active in the human genome. In an invest
igation of the X chromosome, we identified twenty-five SINE-R elements with
between 89.6 and 97.7% homology with the SINE-R.C2 element that is human s
pecific, originally identified in the gene for the C2 component of compleme
nt. SINE-R.C2 and a sequence HS307 that we previously identified in a regio
n of Xq21.3 that has a recently created homology with a 4 Mb block in Yp11.
2 are amongst the group of elements that have diverged furthest from the pa
rent HERV-K10 sequence. The sequences on the X chromosome resemble those th
at we previously described on chromosomes 7 and 17 and the Y chromosome, wi
th a similar range of variation. Phylogenetic analysis from the retroposon
family including those of African great apes using the neighbor-joining met
hod suggests that the SINE-R retroposon family have evolved independently d
uring primate evolution. Further investigation of SINE-R elements on the se
x chromosomes, particularly in recently created regions of X-Y homology, ma
y cast light on the timing of the retroposition process and its possible re
levance to recent evolutionary change.