(Reuters) - For most U.S. families, the holidays are all about gathering around the fireplace, overeating at festive dinners, and playing the occasional board game. But this year, a father and son were also working on the biggest pharmaceutical acquisition of all time.

Clinton Gartin and his son Christopher advised on opposite sides of Bristol-Myers Squibb Co's BMY.N $74 billion deal to buy Celgene Corp CELG.O, which was announced on Thursday.

Clinton Gartin, a chairman of investment banking at Morgan Stanley MS.N, advised Bristol-Myers Squibb. Christopher Gartin, a director in the healthcare investment banking team at Citigroup Inc C.N, advised Celgene, according to people familiar with the deal.

It is rare for a father and son to work as investment bankers on opposite sides of the same deal. Clinton Gartin did not respond to a request for comment on his role, while Christopher Gartin declined to comment through a Citigroup spokeswoman.

Clinton Gartin joined Morgan Stanley in 1981 and became a managing director in 1991. He has held a number of roles within the bank, including chief administrative officer of investment banking, vice chairman of investment banking and global co-head of healthcare.

Christopher Gartin started his career at Lehman Brothers in 2007 and first joined Citigroup in 2008. He left Citigroup for Malin Corporation Plc MLCM.I in 2017, only to rejoin it in 2018.

The Gartins were just two of many bankers advising Bristol-Myers Squibb and Celgene, within their teams and among other investment banks involved.

While Morgan Stanley was the lead financial adviser to Bristol-Myers Squibb, Evercore Inc EVR.N and Dyal Co LLC also advised it, according to the statement announcing the deal. The lead financial adviser to Celgene was JPMorgan Chase & Co JPM.N, and Citigroup acted as an adviser alongside it.

Like all the bankers involved, the Gartins are in line for a big payout if the deal closes. Bristol-Myers Squibb could fork out between $75 million and $85 million to its investment banking advisers, while Celgene could pay between $100 million and $110 million to its investment banking advisers, according to estimates from Freeman & Co.