Mucolipidosis II (ML II), sometimes also referred to as I-cell disease, is a progressively debilitating inherited disorder caused by the accumulation of products throughout the body that are supposed to be broken apart. This is due to a deficient enzyme called G1cNAc-1-phosphotransferase. Affected infants are typically born with several bone abnormalities and most never learn how to walk independently because they have severe joint deformities as well. A few of these skeletal abnormalities can be seen prenatally by ultrasound. They have slow growth through the first year of life and stop growing during the second year of life. Most affected children have heart valve problems and narrow airways that contribute to recurrent respiratory infections. They often have recurrent ear infections, as well, that lead to hearing loss. Their skin becomes thickened and their facial features develop a characteristic look often referred to as "coarse". Motor development is significantly delayed and intellect is also below normal for age. Life expectancy is short, with death typically occurring in childhood. There is no cure for ML II but medical surveillance and care may help to improve some symptoms and overall condition of life. Mucolipidosis II is caused by pathogenic variants in the GNPTAB gene.