Spreading of the interphase nuclei and FISH procedure Using a mouth pipette, the individual blastomeres were first rinsed in medium and then transferred to a 1 µl droplet of 0.01 N HCl/ 0.1% Tween 20 solution (Coonen et al., 1994) on a Superfrost Plus glass slide (Kindler GmbH, Freiburg, Germany). A two-round FISH procedure was performed allowing us to detect the chromosomes X, Y, 13, 18, 21 (round 1) and 16, 22 (round 2) as was previously described in detail (Staessen et al., 2004).