** Shares of biotech firm IGM Biosciences IGMS.O rise 17% to $1.31
** IGMS says it is being bought by Concentra Biosciences for about $74.97 million in cash, according to Reuters calculation
** IGMS shareholders will get $1.247 in cash for each share they own
** IGMS shareholders will also get right for possible extra money if co has more than $82 million in cash at closing or sells certain assets within one year
** Deal expected to close next month
** Wilson Sonsini is IGMS's legal advisor
** Including session's move, stock down 78.7% YTD
(Reporting by Kamal Choudhury in Bengaluru)
((Kamal.Choudhury@thomsonreuters.com;))