Cyclic Neutropenia (CN) is an autosomal recessive stem cell disorder that is fatal. Puppies affected by CN look lighter in colour than non-affected puppies, leading to the term 'Gray Collie Syndrome'.

CN affects the white blood cells (neutrophils) of the affected dog. Around every 10-12 days, the number of neutrophils in the blood stream significantly decreases, which leaves them with a severely compromised immune system.

Symptoms include high susceptibility to infection, diarrhea, fever, bleeding episodes.

Most affected dogs will die as puppies, and are very unlikely to live past the age of 2 years.
 

GENETIC TEST
CN is an autosomal recessive disease which means a dog requires two copies of the mutation for the disease to occur. A genetic test is available to determine carrier status.

The below chart is a theoretical breeding calculator for CN. Results in bold are results where the puppy will display the disease. Results that are 'around' a percentage means that each puppy has the quoted percentage chance of that result. It does not mean that EXACTLY that percentage of the litter will have the result. 
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RESULTS IN PRACTICE
As dogs affected (CN / CN) with Gray Collie Syndrome are unlikely to make it to physical maturity, it is unlikely that one will be bred. However, combinations that can cause affected puppies should never be bred.
 

FURTHER READING
Information on the treatment of a GCS smooth collie here, Bit O' Heaven Collies