Rec8 syndrome (also known as "recombinant 8 syndrome" and "San Luis Valley syndrome") is a chromosomal disorder found in individuals of Hispanic descent with ancestry from the San Luis Valley of southern Colorado and northern New Mexico. Affected individuals typically have mental retardation, congenital heart defects, seizures, a characteristic facial appearance, and other manifestations. The recombinant chromosome is rec(8)dup(8q)inv(8)(p23.1q22.1), and is derived from a parental pericentric inversion, inv(8)(p23.1q22.1). Here we report on the cloning, sequencing, and characterization of the 8p23.1 and 8q22 breakpoints from the inversion 8 chromosome associated with Rec8 syndrome. Analysis of the breakpoint regions indicates that they are highly repetitive. Of 6 kb surrounding the 8p23.1 breakpoint, 75% consists of repetitive gene family members-including Alu, LINE, and LTR elements-and the inversion took place in a small single-copy region flanked by repetitive elements. Analysis of 3.7 kb surrounding the 8q22 breakpoint region reveals that it is 99% repetitive and contains multiple LTR elements, and that the 8q inversion site is within one of the LTR elements.