Complex sentence: The gene encoding myostatin was discovered in 1997 by geneticists Se-Jin Lee and Alexandra McPherron who produced a knockout strain of mice that lack the gene, and have approximately twice as much muscle as normal mice.
Split into simpler sentences: The gene encoding myostatin was discovered in 1997 by geneticists Se-Jin Lee and Alexandra McPherron who produced a strain of mutant mice that lack the gene. These myostatin "knockout" mice have approximately twice as much muscle as normal mice.