What is the statistics of cerebellar hypoplasia in children?

Please be specific: Cerebellar hypoplasia is a descriptive term for many individual entities affecting brain development. It can occur in isolation or part of a complex syndrome. It has been found in cases with linkage to anomalies on the x, 3rd, 7th, 9th, 14th & 17th chromosomes. Their also sibling pairs that define autosomal recessive cases. All are considered rare with outcomes varying to the degree of defect.