## 1 Introduction  
Post translational modification (PTM) is the modification of proteins subsequent to protein biosynthesis. Proteins, undergo PTM to form the anticipated protein product. There are various forms of PTMs, namely hydroxylation, glycosylation, phosphorylation, methylation. ubiquitination, SUMOylation, acetylation and lipidation (Figure 1). Glycoslation is associated with a network of critical biological processes. The process involves attachment of carbohydrate molecules to lipids and protein moieties. Monosaccharides that are covalently linked by glycosidic bonds, are called glycans. N-glycosylation and O-glycosylation are the two major glycosylation types. C-glycosylation(or C-mannosylation), glypiation (or glycosylphosphatidylinositol (GPI) anchoring) and glycation (or nonenzymatic glycosylation) also exist [1,2]. The biological roles of glycosylation include diverse functions and influence protein folding and oligomerization [3], protein degradation [4], epitope recognition [5], protein solubility and stability [6], cell-cell interactions [7] and protein transport [8]. Subsequently, glycoproteins also have a say in cancer development and progression [9], autoimmune diseases [10] and congenital disorders [11]. Tailoring glycosylation for manufacturing biotherapeutics, enhances the safety and efficacy of monoclonal antibodies and erythropoietin. Among all PTMs, glycosylation is established to be the most complex [12]. Biologically active glycan structures are encoded indirectly into the genome and glycosylases, glycosyltransferase and carbohydrate-modifying enzymes help attaching glycan structures to proteins and lipids. [12]. Then the glycoproteins are modified with different carbohydrates resulting in several glycoforms. The recognition of glycan-occupied sites is expensive and laborious and can only be experimentally determined [13]. This is the reason why still the number of verified glycosylated residues are limited as compared to the existing knowledge of the known protein sequences [14]. This knowledge regarding glycosylation site locations is by itself a valuable asset. This knowledge is useful in improving the 3D protein structure prediction, to manipulate protein pharmacokinetic properties through glycoengineering [15]. In this direction, development of bioinformatics tools to predict glycosylation sites gain paramount significance. [PERSON] et al. in their review have discussed that aberrant protein glycosylation can be linked with cancer and protein glycoforms as cancer biomarkers. Aberrant glycosylation can have a say in cancer progression, as glycans do on cell adhesion, migration, immune surveillance, interactions with cell matrix, signaling and cell metabolism [16]. Development and progression of cancer is known to result in fundamental changes to the glycome. Aberrant glycosylation has been reported in prostate cancer, where aberrations in glycan composition of prostate cancer cells are linked to disease progression. Glycan variants have been reported to be associated with breast, colon, liver, skin,  
Figure 1: Schematic showing the wide range of post translational modification of proteins of biological significance. ovarian and bladder cancers and neurodegenerative diseases [17; 18; 19]. Many glycoproteins show alterations of glycosylation during cancer [20], these can act as glycan biomarkers for various cancer types. MUC-1 (CA15-3/CA27.29) [21] and plasminogen activator inhibitor (PAI-1) [22] have been reported as biomarkers for beta-human chorionic gonadotropin (Beta-hCG); breast cancer; [23] testicular cancer; alpha-fetoprotein (AFP) [24] for liver cancer and germ cell tumors. Chromogranin A (CgA) [25] is a biomarker for neuroendocrine tumors and biomarkers MUC16 (CA-125) [26] and HE4 [27] for ovarian cancer. The hallmarks of glycosylation in cancer are presented in Figure 2.  
Bioinformatics resources for glycomics and glycobiology-related databases has been reviewed [28; 29; 30; 31; 32; 33; 34].