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The tumor necrosis factor receptor family, which includes TNF-RI, TNF-R2, Fas, DR3, DR4, DR5, and DR6, plays an important role in the regulation of apoptosis in various physiological systems. The receptors are activated by a family of cytokines that include TNF, FasL, TWEAK, and TRAIL. They are characterized by a highly conserved extracellular region containing cysteine-rich repeats and a conserved intracellular region of about 80 amino acids termed the death domain (DD). The DD is important for transducing the death signal by recruiting other DD containing adaptor proteins (FADD, TRADD, RIP) to the death-inducing signaling complex (DISC) resulting in activation of caspases.

Control of the cell cycle is regulated by a multitude of cellular events and processes. The cyclin dependent kinases (CDK) regulate many of these pathways and constitute an active complex when associated with their cyclin partners. This activity is controlled primarily by phosphorylation, which determines subcellular localization of the CDK/cyclin complex (1,2). Some phosphorylation events control the function of cytoplasmic retention sequences while other events regulate nuclear localization and export sequence function (3,4). Cyclin and cyclin-dependent kinase inhibitor (CKI) levels are regulated by ubiquitination and degradation via the ubiquitin proteasome pathway (5). A variety of CKI proteins associate with these complexes and modulate access to regulatory domains on cyclins (6). Additional complexity is generated as the controlled protein levels of each cyclin oscillate with the stages of cell cycle. Increased expression of Cyclin D1 is associated with certain types of cancer (7,8) and may associate with TSC2 (tuberin) independent of its Cdk partner (9).

Keratins (cytokeratins) are intermediate filament proteins that are mainly expressed in epithelial cells. Keratin heterodimers composed of an acidic keratin (or type I keratin, keratins K9-K28) and a basic keratin (or type II keratin, keratins K1-K8 and K71-K80) assemble to form filaments. Keratin isoforms demonstrate tissue- and differentiation-specific profiles that make them useful as research and clinical biomarkers.

The regulation of cell growth, differentiation and programmed death is coordinated by several sets of proteins that comprise essential signal transduction pathways. Many of these key regulatory proteins are encoded by proto-oncogenes, which can be activated (altered) to change the typical cell program to one of abnormal cell growth and unregulated development. Proteins encoded by proto-oncogenes include growth factors and other ligands, receptor proteins, tyrosine kinases, various regulatory proteins (i.e. GTPases) and transcription factors. Together these proteins comprise the basic elements of cell signaling pathways; altered expression or mutation of one or more of these components can lead to oncogenic growth.

Coronaviruses are a group of viruses that contain single-stranded, positive-sense RNA genomes. Several members of this group, which include severe acute respiratory syndrome coronaviruses (SARS-CoV and SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV), are highly pathogenic and have caused significant disease outbreaks in human hosts. In order for human coronaviruses to transcribe and replicate their genomes within host cells, they must first attach and gain entry into host cells using a variety of cell surface receptors and components of the endocytic machinery.

Formation of Cleaved Caspase: During apoptosis, caspase zymogens are cleaved to form active enzymes. This cleavage is irreversible and marks the irreversible stage of apoptosis.In many diseases, abnormal regulation of apoptosis is closely related to the occurrence and development of the disease.

Mammalian histone deacetylases can be divided into three classes on the basis of their similarity to various yeast deacetylases. Class I proteins (HDACs 1, 2, 3, and 8) are related to the yeast Rpd3-like proteins, those in class II (HDACs 4, 5, 6, 7, 9, and 10) are related to yeast Hda1-like proteins, and class III proteins are related to the yeast protein Sir2. Inhibitors of HDAC activity are now being explored as potential therapeutic cancer agents.

Cellular Localization IF (Immunofluorescence) is an experimental method based on immunofluorescence technology, used to observe the distribution of specific antigens within cells.

Cdc25 is a protein phosphatase responsible for dephosphorylating and activating cdc2, a crucial step in regulating the entry of all eukaryotic cells into mitosis (1). cdc25C is constitutively phosphorylated at Ser216 throughout interphase by c-TAK1, while phosphorylation at this site is DNA damage-dependent at the G2/M checkpoint (2). When phosphorylated at Ser216, cdc25C binds to members of the 14-3-3 family of proteins, sequestering cdc25C in the cytoplasm and thereby preventing premature mitosis (3). The checkpoint kinases Chk1 and Chk2 phosphorylate cdc25C at Ser216 in response to DNA damage (4,5).

c-Kit is a member of the subfamily of receptor tyrosine kinases that includes PDGF, CSF-1, and FLT3/flk-2 receptors. It plays a critical role in activation and growth in a number of cell types, including hematopoietic stem cells, mast cells, melanocytes, and germ cells.