In a monozygotic twin couple with mental retardation (MR), we identified a maternally inherited inversion and a paternally inherited translocation: 46,XY,inv(10)(p11.2q21.2)mat,t(9;18)(p22;q21.1)pat. The maternally inherited inv(10) was a benign variant without any apparent phenotypical implications. The translocation breakpoint at 9p was within a cluster of interferon α genes and the 18q21 breakpoint truncated ZBTB7C (zinc finger and BTB containing 7C gene). In addition, analyses with array-CGH revealed a 931 kb maternally inherited deletion on chromosome 8q22 as well as an 875 kb maternally inherited duplication on 5p14. The deletion encompasses the RIM2 (Rab3A-interacting molecule 2), FZD6 (Frizzled homolog 6) and BAALC (Brain and Acute Leukemia Gene, Cytoplasmic) genes and the duplication includes the 5' end of the CDH9 (cadherin 9) gene. Exome sequencing did not reveal any additional mutations that could explain the MR phenotype. The protein products of the above mentioned genes are involved in different aspects of brain development and/or maintenance of the neurons which suggest that accumulation of genetic defects segregating from both parents might be the basis of MR in the twins. This hypothesis was further supported by protein interaction analysis.