-DS complex phenotype results from dosage imbalance of genes located on human chromosome 21(Hsa 21). The genetic nature of DS together with the relatively small size of Hsa 21 encouraged scientist to concentrate efforts towards the complete characterization of this chromosome in the past few years. The length of 21q is 33.5 Mb [11] and 21 p is 5–15 Mb [12]. A total 225 genes was estimated when initial sequence of 21q was published [11]. Hsa 21 has 40.06% repeat content out of which the repeat content of SINE’s, LINE’s, and LTR are 10.84%, 15.15%, 9.21% respectively. The Table 1 given below highlights some of the genes present on chromosome 21.