bovine presents a complex genome due to high number of repetitive sequences involved in macro and micro-rearrangements that are essential in the bovids evolution. however, it has a morphologically simple karyotype, with acrocentric autosomes (2n=60,xx and 60,xy). the robertsonian translocation rob(1;29), which produces embryo mortality, has been widely described in different breeds of bos taurus but the complexity of its mechanism and its consequences in chromatin changes, are still not clear. clastogenic agents are used as an approach to study this rearrangement in chromatin structure. particularly, the aphidicolin (apc) inhibits the eukaryotic dna polymerase a during replication. this fact permit to identify regions rich in dctp due to a competition and spreading of the enzyme. lymphocyte macro culture of 2 female uruguayan creole cattle carriers of rob(1;29) were apc-inducted (0,3um) in one cell cycle. a new break point, rob(1;29)p13/21 (relative distance: p2c/p1=0.45), is located in a late replication region (rbg- band). two fragile sites (c-fra) in the long arm of rob(1;29) (q13/ 21 and q43) and two other c-fra in btax (q12 and q31) were scored. c-fra: rob(1:29)q13/21 y xq12 were located proximal to centromere in later replication region while rob(1;29)q43 and xq31 were located proximal to telomere in early replication region. a high incidence in c-fra proximal to centromere in both biarmed chromosomes (btaxc2=11.3 p<0.001; rob(1;29)c2=4.66p<0.05) was found. comparing apc-effect: bta1q13/21 and rob(1;29)q13/21, the high damage found in the second was equivalent to 75% of heterozygosis. this region (rbg- band) has a microsatellite associated to this rearrangement and tissue specific genes.