in C. elegans. Thus, the function of the selective silencing may be to spare dosage compensation of genes crucial for sex determination of homologous pairing. While the epigenetic mechanisms behind dosage compensation in birds are poorly understood, especially in comparison to the well-studied mechanisms of dosage compensation in humans and Drosophila, several recent studies have revealed promising sequences. One example is MHM (male hypermethylated) RNA, an Xist-like long noncoding RNA that is expressed only in female chickens (ZW). It is associated with female-specific hyper-acetylation of lysine 16 on histone 4 near the MHM locus on the Z chromosome. This MHM locus is heavily studied as a site of dosage compensation because male Z chromosomes are hypermethylated and thus underexpress genes in this area in comparison to female Z chromosomes which are hyperacetylated and overexpress these genes. There has been debate on whether the MHM locus constitutes dosage compensation, however, since scientists claim that even if the MHM locus has been found to have significantly greater expression in females than in males, it could not even be considered to be a dosage compensation mechanism since it does not balance gene dose between the Z chromosome and autosomes in the heterogametic sex. Similar to mammals, chickens seem to use CpG islands (segments of Cytosine-phosphate-Guanine that are more readily methylated and silenced than other DNA segments) to regulate gene expression. One study found that CpG islands were found primarily in compensated areas of the Z chromosome—areas that are differentially expressed in male and female chickens. Thus it is likely that these CpG islands are locations where genes on the male Z chromosome are methylated and silenced, but which stay functional on the female Z chromosome. Monotremes Monotremes are a class of basal mammals that also lay eggs. They are an order of mammals that includes platypuses and four species of echidna, all of which are egg-laying mammals. While monotremes use an XX/XY system, unlike other mammals, monotremes have more than two sex chromosomes. The male short-beaked echidna, for example, has nine sex chromosomes—5 Xs and 4 Ys, and the male platypus has 5 Xs and 5 Ys. Platypuses are a monotreme species whose mechanism of sex determination has been extensively studied. There is some contention in academia about the evolutionary origin and the proper taxonomy of platypuses. A recent study revealed that four platypus X chromosomes, as well as a Y chromosome, are