Examples of FISH analysis performed in case 18. Arrowheads in a and c indicate normal chromosome 21 that cohybridize with pUC 1.76 probe for the centromere of chromosome 13. Probes applied: (a) pUC 1.76 (cen 13/21) (red) and 911F6 (green). The picture shows a metaphase with two copies of normal chromosome 13 and one der(13): YAC 911F6 hybridizes on both normal 13 and on the der(13); (b) pBR-12 (cen-12) (green) and RB1 (red). Only one red signal, corresponding to RB1 gene, is kept on normal 13; (c) pUC 1.76 (red) and 937C7 (green). This metaphase represents the subclone with one normal 13 and two copies of der(13): YAC 937C7 is only mantained on normal 13; (d) pBR-12 (red) and 935G2 (green) in a metaphase with two copies of normal 13 and one der(13): YAC935G2 is present in three copies, on the two normal 13 and on the der(12).