Replication patterns of the duplicate-type sex chromosomes have been studied in bone-marrow cells in vivo and fetal cells in vitro of the Indian house shrew, Suncus murinus. The sex chromosomes start their replication along with the rest of the chromosomes in the early S but show asynchrony in the late S phase. The Y chromosome finishes its DNA synthesis earlier than the X chromosome and the autosomes in male bone-marrow and fetal cells. On the other hand, the X chromosomes in female cells behave differently in bone-marrow and fetal cells. One of the X’s replicates early in fetal cells. But there is no definite pattern in bone-marrow cells – the X chromosomes can replicate either early or late. The duplicate-type X chromosome of Suncus is unique among mammals in its replication; the pattern does not conform to the hypothesis that except 5% of the haploid complement residing in the X chromosome is functional. The correlation between genetic activity and late or early replication is discussed. It is proposed that heteropycnosis is merely a reflection of out-of-phase replication and not necessarily genetic inertn