Russians who have one grandparent Jewish, are able to move to Israel under the law of Return. This was an allowance because of the rife Antisemitism in the Soviet Union where people would hide their Jewishness or actively encourage their children to marry outside the tribe to avoid persecution.

For a great deal of those Russians, they are not Jewish by halacha, cannot marry within Israel etc. For then to actually become a member of the tribe, they have to go through the same ardous process, not be already married, and have to attend a Beth Din court and get grilled on the finer aspects of Jewish religious law, culture, language and history.