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

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

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 reconnaissance biplane
• Hansa-Brandenburg C.I, a 1916 German 2-seater armed single-engine reconnaissance biplane
• Kawasaki C-1, an indigenous transport aircraft of the Japanese Air Self-Defense Forces
• Otto C.I, a 1915 German reconnaissance biplane
• Rumpler C.I, a 1915 German two-seater single-engine reconnaissance biplane
• Southern Cross Airport (New Jersey) (FAA LID: C01)

Land

Rail

• Bavarian C I, an 1847 German steam locomotives class
• GNR Class C1 (small boiler), a British 4-4-2 steam locomotive class (classified C2 during LNER ownership)
• GNR Class C1 (large boiler), a British 4-4-2 steam locomotive class
• LB&SCR C1 class, an 1882 British freight steam locomotive
• Long Island Rail Road C1, a bilevel railcar built by Tokyu Car Corporation
• NER Class C1, a British 0-6-0 steam locomotive class
• PRR C1, an American PRR steam locomotive
• Finnish Steam Locomotive Class C1

Sea

• HMS C1, a C-class submarine of the Royal Navy
• USS C-1 (SS-9), a C-class submarine of the United States Navy
• USS Newark (C-1), a protected cruiser of the United States Navy
• Type C1 ship, a small cargo vessel built by the United States in large numbers before and during World War II
• Isaac Peral (C-1), a Spanish submarine

Space

• Cluster 1, also known as Rumba, an ESA satellite