of 70 years-old men to some degree, contributes to fibrosis, heart risks, and mortality in a causal way. Male smokers have between 1.5 and 2 times the risk of non-respiratory cancers as female smokers. Potential countermeasures identified so far include not smoking or stopping smoking and at least one potential drug that "may help counteract the harmful effects of the chromosome loss" is under investigation. Structure Cytogenetic band Non-combining region of Y (NRY) The human Y chromosome is normally unable to recombine with the X chromosome, except for small pieces of pseudoautosomal regions (PARs) at the telomeres (which comprise about 5% of the chromosome's length). These regions are relics of ancient homology between the X and Y chromosomes. The bulk of the Y chromosome, which does not recombine, is called the "NRY", or non-recombining region of the Y chromosome. Single-nucleotide polymorphisms (SNPs) in this region are used to trace direct paternal ancestral lines. More specifically, PAR1 is at 0.1–2.7 Mb. PAR2 is at 56.9–57.2 Mb. The non-recombining region (NRY) or male-specific region (MSY) sits between. Sequence classes Genes Number of genes The following are some of the gene count estimates of human Y chromosome. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies (for technical details, see gene prediction). Among various projects, the collaborative consensus coding sequence project (CCDS) takes an extremely conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes. Gene list In general, the human Y chromosome is extremely gene poor—it is one of the largest gene deserts in the human genome. Disregarding pseudoautosomal genes, genes encoded on the human Y chromosome include: Y-chromosome-linked diseases Diseases linked to the Y chromosome typically involve an aneuploidy, an atypical number of chromosomes. Y chromosome microdeletion Y chromosome microdeletion (YCM) is a family of genetic disorders caused by missing genes in the Y chromosome. Many affected men exhibit no symptoms and lead normal lives. However, YCM is also known to be present in a significant number of men with reduced fertility or reduced sperm count. Defective Y chromosome This results in the person presenting a female phenotype (i.e., is born with female-like genitalia) even though that person possesses an XY karyotype. The lack of the second X results in infertility. In other words, viewed from the opposite direction, the person goes