Genetics and knockout mice?

[midnightspirirt]

Gene “X” is specifically expressed in the central nervous system. When “X” was knocked out in mice, no morphological & physiological phenotype was detectable. What is your interpretation for this result? How would you design the next knockout experiment in order to understand the role of gene “X”?

If someone can explain the first part of the question, I would highly appreciate it so I can figure out the rest.
Do you think the question is referring to a single or double knockout with genes?