C1, C01, C.I or C-1 may refer to: Arts and media C1, a note-octave in music C1 Television, a Mongolian television channel Schecter C-1 Hellraiser FR, a guitar model A Yamaha grand piano model "C1", a slang expression in the video game Counter-Strike, used to express agreement Biology and medicine Anatomy Cervical vertebra 1, the first cervical vertebrae of the vertebral column Cervical spinal nerve 1, a spinal nerve of the cervical segment Biochemistry C1 complex, the first component of the classical complement pathway C1 domain, an important secondary messenger protein domain C1-inhibitor, a human serine protease inhibitor C1 regulatory sequence for the insulin gene Apolipoprotein C1, a human lipoprotein Chlorophyll c1, a form of chlorophyll Cytochrome C1, a precursor protein to Cytochrome C Proanthocyanidin C1, a type of polyphenolic compound Prostaglandin C1, a form of prostaglandins Other uses in biology and medicine C1 and P1 (neuroscience), a component of the visual evoked potential ATC code C01, a subgroup of the Anatomical Therapeutic Chemical Classification System C01, the ICD-10 code for "malignant neoplasm of base of tongue", a form of oral cancer C1, the short name for a microfluidic single-cell analysis system sold by Fluidigm Chemistry C1 Chemistry, study of methane, methanol, carbon monoxide, carbon dioxide Computing Hardware C-One, a single-board microcomputer C1 NES TV, a family of Nintendo-television-set hybrids manufactured by Sharp Corporation in 1983 ATI C1, codename of the Xbox 360 GPU Xenos Other uses in computing C0 and C1 control codes defined in ISO 6429 C1 (protocol), a 1984 protocol for file transfer, also known as "Punter" TCSEC C1 security class A CPU power state in the Advanced Configuration and Power Interface Mathematics , the set of all continuously differentiable functions C1 field, a quasi-algebraically closed field C1, the first of four pure modules taken in the Advanced Level (UK) Maths syllabus Transportation Air C-1 Trader, a cargo-version of the S-2 Tracker anti-submarine warfare aircraft of the United States Navy AEG C.I, a German World War I reconnaissance aircraft AGO C.I, a 1915 German reconnaissance biplane Albatros C.I, a 1915 German two-seat general-purpose biplane Aviatik C.I, a 1915 German observation plane Cierva C.1, a 1920 Spanish experimental autogyro DFW C.I, a WWI German reconnaissance aircraft Douglas C-1, a cargo/transport airplane produced by Douglas Aircraft for the U.S. Army Air Service starting in 1925 Fokker C.I, a 1918 German reconnaissance biplane Friedrichshafen C.I, a 1914 German single-engined amphibious