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For the Miles Davis album, see Relaxin' with the Miles Davis Quintet.
Relaxin 1
Symbol RLN1
Alt. symbols H1
Entrez 6013
HUGO 10026
OMIM 179730
RefSeq NM_006911
UniProt P04808
Other data
Locus Chr. 9 qter-q12
Relaxin 2
Symbol RLN2
Alt. symbols H2, RLXH2, bA12D24.1.1, bA12D24.1.2
Entrez 6019
HUGO 10027
OMIM 179740
RefSeq NM_134441
UniProt P04090
Other data
Locus Chr. 9 qter-q12
Relaxin 3
Symbol RLN3
Alt. symbols ZINS4, RXN3, H3
Entrez 117579
HUGO 17135
OMIM 606855
RefSeq NM_080864
UniProt Q8WXF3
Other data
Locus Chr. 19 p13.3

Relaxin is a peptide hormone that was first described in 1926 by Frederick Hisaw.[1 ][2]

The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1 (RNL1), 2 (RNL2) and 3( RNL3), and the insulin-like (INSL) peptides, INSL3, INSL4, INSL5 and INSL6. The functions of relaxin-3, INSL4, INSL5, INSL6 remain uncharacterised.[3]



In the female, it is produced by the corpus luteum of the ovary, the breast and, during pregnancy, also by the placenta, chorion, and decidua.

In the male it is produced in the prostate and is present in human semen.[4]


Structurally, relaxin is a heterodimer of two peptide chains of 24 and 29 amino acids that are linked by disulfide bridges and it appears related to insulin.

Relaxin is produced from its prohormone, “pro-relaxin”, by splitting off one additional peptide chain.


Enhances motility of sperm in semen.[5]

In humans

Relaxin is produced mainly by the corpus luteum, in both pregnant and non-pregnant females, it rises to a peak within approximately 14 days of ovulation and then declines in the absence of pregnancy resulting in menstruation. During the first trimester of pregnancy levels rise and additional relaxin is produced by the decidua.

Relaxin's role or necessity in human pregnancy remains under investigation, as in humans its peak is reached during the 14 weeks of the first trimester and at delivery. It is believed to soften pubic symphysis.

In non-human animals

In animals relaxin widens the pubic bone and facilitates labor, it also softens the cervix (cervical ripening), and relaxes the uterine musculature. Thus, for a long time, relaxin was looked at as a pregnancy hormone. However, its significance may reach much further. Relaxin affects collagen metabolism, inhibiting collagen synthesis and enhancing its breakdown by increasing matrix metalloproteinases.[6] It also enhances angiogenesis and is a potent renal vasodilator.


Relaxin interacts with the relaxin receptor LGR7 (RXFP1) and LGR8 (RXFP2) which belong to the G-protein-coupled receptor superfamily. They contain a heptahelical transmembrane domain and a large glycosylated ectodomain, distantly related to the receptors for the glycoproteohormones, such as the LH-receptor or FSH-receptor.

Relaxin receptors have been found in the heart, smooth muscle, the connective tissue, and central and autonomous nervous system.


Specific disorders related to relaxin have not been described, yet it has been suggested that it could be linked to scleroderma and to fibromyalgia.[7]


  1. ^ "If a Gopher Can Do It ...". Time Magazine. 1944-04-10.,9171,796530,00.html. Retrieved 2009-05-20.  
  2. ^ Becker GJ, Hewitson TD (March 2001). "Relaxin and renal fibrosis". Kidney Int. 59 (3): 1184–5. doi:10.1046/j.1523-1755.2001.0590031184.x. PMID 11231378.  
  3. ^ Wilkinson TN, Speed TP, Tregear GW, Bathgate RA (February 2005). "Evolution of the relaxin-like peptide family". BMC evolutionary biology 5 (1): 14. doi:10.1186/1471-2148-5-14. PMID 15707501.  
  4. ^ MacLennan AH (1991). "The role of the hormone relaxin in human reproduction and pelvic girdle relaxation". Scandinavian journal of rheumatology. Supplement 88: 7–15. PMID 2011710.  
  5. ^ Weiss G (February 1989). "Relaxin in the male". Biol. Reprod. 40 (2): 197–200. doi:10.1095/biolreprod40.2.197. PMID 2497805.  
  6. ^ Mookerjee I, Solly N, Royce S, Tregear G, Samuel C, Tang M (2006). "Endogenous relaxin regulates collagen deposition in an animal model of allergic airway disease". Endocrinology 147 (2): 754–61. doi:10.1210/en.2005-1006. PMID 16254028.  
  7. ^ Van Der Westhuizen E, Summers R, Halls M, Bathgate R, Sexton P (2007). "Relaxin receptors--new drug targets for multiple disease states". Curr Drug Targets 8 (1): 91–104. doi:10.2174/138945007779315650. PMID 17266534.  

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