Other research has suggested that a greater than average number of structural variations such as rare deletions or duplications of tiny DNA sequences within genes (known as copy number variations) are linked to increased risk for schizophrenia, especially in "sporadic" cases not linked to family history of schizophrenia, and that the genetic factors and developmental pathways can thus be different in different individuals. A genome wide survey of 3,391 individuals with schizophrenia found CNVs in less than 1% of cases. Within them, deletions in regions related to psychosis were observed, as well as deletions on chromosome 15q13.3 and 1q21.1.