Using SPSS for Kaplan Meier not detecting levels of exposure variable

I have a data set of 32 patients with different gene expressiones (exposure) codified as 0 negative and 1 positive. Some of them have missing values (blank). When I try to run the survival analysis SPSS will not consider 0 or 1 and just takes into account the missing values. I tried deleting the subjects with missing values but then the analysis is not conducted at all since no levels on the variable are found.
I am a SAS user so I'm having a little trouble understanding SPSS, I don't know if there is a something like a "CLASS" statement?.