Sequence changes can be very complex, involving a range of changes at one specific location. Complex changes, including translocations, are described using the recommendations of the accepted HGVS nomenclature named extension ISCN, based on the original proposal SVD-WG004 (ISCN<>HGVS)). The named ISCN extension has been developed in collaboration with Standing Committee on Human Cytogenomic Nomenclature (ISCN), covering the description of numerical and structural chromosomal changes detected using microscopic and cytogenetic techniques. It should be noted there is a basic difference between ISCN and HGVS: while ISCN describes the structure of the resulting chromosome(s), HGVS describes the variant(s) detected. It should be noted that the description of complex changes can become rather complicated and at some point, although literally correct, becomes effectively meaningless.
The named ISCN extension has been introduced in 2016 and was modified last in May 2020.
The description of translocations has changed
In the original proposal (SVD-WG004) one identical derivative chromosome would receive two different descriptions, depending on whether it was identified in a balanced or an unbalanced case. In a balanced case the description would use a “::” format joining the breakpoints, while in an unbalanced case the description would use a “delins” format. HGVS recommendations try to avoid such conflicts wherever possible. HGVS therefore recommends to describe translocations exclusively using a “delins” format.