Variable properties - missing

In the Variable View, use the missing cell to enter missing value codes. These are values defined as containing missing data and are designated as user-missing values. Click in the missing area of any variable. This brings up the ellipsis button. Click in the button to open the Missing Values dialog box:

By default, there are no defined missing values. Using the dialog box, you can define up to three explicit missing values, or you can specify two data codes that are endpoints of a range along with an optional discrete missing value. The purpose of allowing multiple missing value codes is to accommodate different mechanisms for missingness.

The equivalent syntax is the MISSING VALUES command.