Moral: How to handle missing values when using LIST input
Use a period (.) as place holder

DATALINES;  
1 68 144 M 23  
2  . 202 M 34  
3 62 99 F 37  
4 61 101 F 45  
;

Note that the period is separated by the other fields with a blank and that there is one period per missing value.
Do not code missing values as some arbitrary value such as: 0 (zero) or 999.