Regression is a term for the loss of previously acquired developmental skills in an individual. This can be in the areas of daily living, language, motor abilities/function, or social interaction. Regression can occur, over weeks to months, or more quickly and time course may help in determining the likely cause of the regression. Regression can be caused by many things and is associated with a marked decline in previously established function. Regression can also be referred to Down syndrome regression disorder (DSRD), Down syndrome disintegrative disorder (DSDD) or unexplained regression in Down syndrome (URDS) and these terms are sometimes used interchangeably.