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England pair Billy and Mako Vunipola have signed new deals with Saracens through to 2019.
The Allianz Park club announced the contract extensions on Monday afternoon, following on from new deals for Jamie George and Duncan Taylor.
Mako Vunipola joined Sarries from Bristol in 2011, with brother Billy following from Wasps two years later.
Saracens rugby director Mark McCall hailed the Vunipolas' progress from raw talents to "two of the world's best".
"This is fantastic news for everyone associated with the club," said McCall.
"To have two players of the calibre of Mako and Billy commit their futures to Saracens is extremely exciting, as we continue to build a strong squad for the upcoming seasons.
"We have a great blend of youth and experience at the club and what has been great is for us to witness the progression of both of them from promising youngsters to two of the world's best in their respective positions."
When Former Tonga captain Fe'a Vunipola and his wife Iesinga, a methodist minister, settled in Wales, their two sons quickly made strides in junior rugby.
Prop Mako excelled at Millfield School and number eight Billy thrived at Harrow, as both won scholarships to top English schools.
The bustling duo breezed through England's age-group sides before announcing their Test readiness in 2012 and 2013.
While Mako broke through at Saracens, so Billy was shining for Wasps. Family ties eventually reunited the pair in north London, and the siblings have since become mainstays for both Saracens and England.
"Signing a new contract comes with new responsibility and I am really looking forward to contributing to Saracens in the upcoming years," said Mako.
"We're extremely happy to extend our stay at the club, as we have made so many fantastic memories together already.
"One of the most important things is that we're playing alongside our best mates, week-in-week-out, and the whole culture of the club means it's a special place to play rugby. I'm delighted to be staying."
Billy Vunipola quickly targeted helping Aviva Premiership table-toppers Saracens extend their domestic dominance after completing his new deal.
"We are both delighted to be able to extend our deals," said Billy.
"Now is time to build on the work we have already done with Saracens and continue to improve as people and players.
"We have a lot to look forward to over the rest of the season and beyond.
"We're very excited to stay at Saracens."
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