Complex sentence: In 2002, Panda generated a line of melanopsin knockout (KO) mice using homologous recombination in mice embryonic stem cells by replacing the first exon of the melanopsin gene with a neomycin-resistant gene.
Split into simpler sentences: In 2002, Panda generated a line of melanopsin knockout (KO) mice through homologous recombination in mice embryonic stem cells. In these mice, they replaced the first exon of the melanopsin gene with a neomycin-resistant gene.