Catgirl Syndrome (カトルスシンドローム) is an unexplained illness that primarily affects girls aged 0 to 12, causing them to suddenly transform into 'catgirls.' There are two things that modern medicine has clarified about it. One is that they are 'cute.' The other is... that they 'get butterflies when they see girls.' (Author: Nekotarou / First published: GANMA! Episodes 83 to 100)