# Gene features and anchor points¶

There are several immunologically important parts of TCR/BCR gene (gene features). For example, such regions are three complementarity determining regions (CDR1, CDR2 and CDR3), four framework regions (FR1, FR2, FR3 and FR4) etc.

The key feature of MiXCR is the possibility to specify:

For convenience, in MiXCR these regions can be specified in terms of above mentioned immunological gene features. The illustrated list of predefined gene features can be found below. The set of possible gene regions is not limited by this list:

• boundary points of gene features (called anchor points) can be used to specify begin and end of custom gene regions
• gene features can be concatenated (e.g. VTranscript = {V5UTRBegin:L1End}+{L2Begin:VEnd}).
• offsets can be added or subtracted from original positions of anchor points to define even more custom gene regions (for more detailed description see gene feature syntax)

Naming of gene features is based on IMGT convention described in Lefranc et al. (2003), Developmental & Comparative Immunology 27.1 (2003): 55-77.

## Germline features¶

Features defined for germline genes are mainly used in align and export.

### V Gene structure¶

Additionally to core gene features in V region (like FR3) we introduce VGene, VTranscript and VRegion for convenience.

## Mature TCR/BCR gene features¶

Features described here (like CDR3) cannot not be used for align, since they are not defined for germline genes.

### V(D)J junction structure¶

Important difference between rearranged TCR/BCR sequence and germline sequence of its segments lies in the fact that during V(D)J recombination exact cleavage positions at the end of V gene, begin and end of D gene and begin of J gene varies. As a result in most cases actual VEnd, DBegin, DEnd and JBegin anchor positions are not covered by alignment:

In order to use actual V, D, J gene boundaries we introduce four additional anchor positions: VEndTrimmed, DBeginTrimmed, DEndTrimmed and JBeginTrimmed and several named gene features: VDJunction, DJJunction and VJJunction. On the following picture one can see the structure of V(D)J junction:

If D gene is not found in the sequence or is not present in target locus (e.g. TRA), DBeginTrimmed and DEndTrimmed anchor points as well as VDJunction and DJJunction gene features are not defined.

Similar to ...Trimmed anchor points in V(D)J junction there is a V5UTRBeginTrimmed anchor point representing left bound of alignment upstream start codon. This point is required because 5’UTR could have different length from transcript to transcript, and because library of gene segments inside MiXCR does’n have information on exact 5’UTR lengths.

## Gene feature syntax¶

Syntax for gene features is the same everywhere. The best way to explain it is by example:

• to enter any gene feature mentioned above or listed in the next section just use its name: VTranscript, CDR2, V5UTR etc.
• to define a gene feature consisting of several concatenated features use +: V5UTR+L1+L2+VRegion is equivalent to VTranscript
• to create gene feature starting at anchor point X and ending at anchor point Y use {X:Y} syntax: {CDR3Begin:CDR3End} for CDR3.
• one can add or subtract offset from original position of anchor point using positive or negative integer value in brackets after anchor point name AnchorPoint(offset): {CDR3Begin(+3):CDR3End} for CDR3 without first three nucleotides (coding conserved cysteine), {CDR3Begin(-6):CDR3End(+6)} for CDR3 with 6 nucleotides downstream its left bound and 6 nucleotides upstream its right bound.
• one can specify offsets for predefined gene feature boundaries using GeneFeatureName(leftOffset, rightOffset) syntax: CDR3(3,0), CDR3(-6,6) - equivalents of two examples from previous item
• all syntax constructs can be combined: {L1Begin(-12):L1End}+L2+VRegion(0,+10)}.

## List of predefined gene features¶

Gene Feature Name Gene feature decomposition
V5UTRGermline {UTR5Begin:V5UTREnd}
VGene {UTR5Begin:VEnd}
VTranscript {UTR5Begin:L1End}+{L2Begin:VEnd}
VDJTranscript {UTR5Begin:L1End}+{L2Begin:FR4End}
V5UTR {V5UTRBeginTrimmed:V5UTREnd}
VDJTranscriptWithout5UTR {L1Begin:L1End}+{L2Begin:FR4End}
VLIntronL {L1Begin:L2End}
L {L1Begin:L1End}+{L2Begin:L2End}
VTranscriptWithout5UTR {L1Begin:L1End}+{L2Begin:VEnd}
Exon1 {L1Begin:L1End}
Intron {VIntronBegin:VIntronEnd}
Exon2 {L2Begin:FR4End}
VExon2Trimmed {L2Begin:VEndTrimmed}
L2 {L2Begin:L2End}
VExon2 {L2Begin:VEnd}
VRegionTrimmed {FR1Begin:VEndTrimmed}
VDJRegion {FR1Begin:FR4End}
FR1 {FR1Begin:FR1End}
VRegion {FR1Begin:VEnd}
CDR1 {CDR1Begin:CDR1End}
FR2 {FR2Begin:FR2End}
CDR2 {CDR2Begin:CDR2End}
FR3 {FR3Begin:FR3End}
GermlineVCDR3Part {CDR3Begin:VEnd}
VCDR3Part {CDR3Begin:VEndTrimmed}
CDR3 {CDR3Begin:CDR3End}
ShortCDR3 {CDR3Begin(3):CDR3End(-3)}
VJJunction {VEndTrimmed:JBeginTrimmed}
VDJunction {VEndTrimmed:DBeginTrimmed}
DRegion {DBegin:DEnd}
DCDR3Part {DBeginTrimmed:DEndTrimmed}
DJJunction {DEndTrimmed:JBeginTrimmed}
JRegion {JBegin:FR4End}
GermlineJCDR3Part {JBegin:CDR3End}
JCDR3Part {JBeginTrimmed:CDR3End}
JRegionTrimmed {JBeginTrimmed:FR4End}
FR4 {FR4Begin:FR4End}
CRegion {CBegin:CEnd}
CExon1 {CBegin:CExon1End}