HTT and Huntington disease: To further investigate the effects of the pathogenic HD mutation on these functions, we compared mhtt knock-in ESCs, hereby referred to as Q111 ESCs, which carries an expanded polyglutamine tract (111 glutamines), with wild type htt knock-in ESCs, hereby referred to as Q18, which conversely carries a normal polyglutamine tract (18 glutamines) [15,16].