Specifications

Fastq sequence description

Fields in fastq description:

Key	Description
instrument	Instrument ID
run_number	Run number on instrument.
flowcell_ids	Flowcell Identifier
flowcell_ids	Flowcell IDS
lane	Lane number
tile	Tile number
x_pos	Position X of cluster
y_pos	Position Y of cluster
UMI	Optional, appears when UMI is specified in sample sheet. UMI sequences for Read 1 and Read 2, seperated by a plus [+]
read	Read number - 1 can be single read or Read 2 of paired-end
is_filtered	Y if the read is filtered (did not pass), N otherwise
control_number	0 when none of the control bits are on, otherwise it is an even number. On HiSeq X and NextSeq systems, control specification is not performed and this number is always 0.
index	Index of the read

See also https://help.basespace.illumina.com/files-used-by-basespace/fastq-files

Filter file

The filter files can be found in the BaseCalls directory. The filter file specifies whether a cluster passed filters. Filter files are generated at cycle 26 using 25 cycles of data. For each tile, one filter file is generated. Location: Data/Intensities/BaseCalls/L001 File format: s_[lane]_[tile].filter

The format is described below

Bytes	Description
0-3	Zero value (for backwards compatibility)
4-7	Filter format version number
8-11	Number of clusters
12-(N+11)	Where N is the cluster number. unsigned 8-bits integer Bit 0 is pass or failed filter

Filter bytes example:

bytes([0, 0, 0, 0]) # prefix 0
bytes([3, 0, 0, 0]) # version 3
struct.pack("<I", cluster_count) # number of cluster in little endian unsigned int
bytes([1]*cluster_count) # For each cluster an unsigned 8-bits integer Where Bit 0 is pass or failed filter

1 == PASS FILTER
0 == NO PASS FILTER

In hexdump:

BYTES 0-3      BYTES 4-7      BYTES 8-11     BYTES 12-14
00 00 00 00    03 00 00 00    03 00 00 00    01 01 01

At bytes 8-11 I have 3 clusters and each cluster is represented by a an unsigned 8-bit integer.

Control file

The control files are binary files containing control results.

Bytes	Description
0-3	Zero value (for backwards compatibility)
4-7	Format version number
12-(2xN+11)	Where N is the cluster number Bit 0: always empty (0) Bit 1: was the read identified as a control? Bit 2: was the match ambiguous? Bit 3: did the read match the phiX tag? Bit 4: did the read align to match the phiX tag? Bit 5: did the read match the control index sequence? Bits 6,7: reserved for future use Bits 8..15: the report key for the matched record in the controls.fasta file (specified by the REPORT_KEY metadata)

Locations file

The BCL to FASTQ converter can use different types of position files and will expect a type based on the version of RTA used The locs files can be found in the Intensities/L<lane> directories

Bcl file

The BCL files can be found in the BaseCalls directory inside the run directory: Data/Intensities/BaseCalls/L<lane>/C<cycle>.1

They are named as follows:

s_<lane>_<tile>.bcl

Format:

Bytes	Description
0-3	Number of N clusters in unsigned 32bits little endian integer
4-(N+3)	Unsigned 8 bits integer Bits 0-1 are bases encoded as: [A,C,G,T] -> [0,1,2,3] -> [00,01,10,11] Bits 2-7 are shifted by 2 bits and contain the quality score. All bits ‘0’ is reserved for no call (N)

Stat file

The stats files can be found in the BaseCalls directory inside the run directory: Data/Intensities/BaseCalls/L00<lane>/C<cycle>.1

They are named as follows:

s_<lane>_<tile>.stats

The Stats file is a binary file containing base calling statistics; the content is described below.

The data is for clusters passing filter only:

Start	Description	Data type
Byte 0	Cycle number	integer
Byte 4	Rverage Cycle Intensity	double
Byte 12	Average intensity for A over all clusters with intensity for A	double
Byte 20	Average intensity for C over all clusters with intensity for C	double
Byte 28	Average intensity for G over all clusters with intensity for G	double
Byte 44	Average intensity for A over clusters with base call A	double
Byte 52	Average intensity for C over clusters with base call C	double
Byte 60	Average intensity for G over clusters with base call G	double
Byte 68	Average intensity for T over clusters with base call T	double
Byte 76	Number of clusters with base call A	integer
Byte 80	Number of clusters with base call C	integer
Byte 84	Number of clusters with base call G	integer
Byte 88	Number of clusters with base call T	integer
Byte 92	Number of clusters with base call X	integer
Byte 96	Number of clusters with intensity for A	integer
Byte 100	Number of clusters with intensity for C	integer
Byte 104	Number of clusters with intensity for G	integer
Byte 108	Number of clusters with intensity for T	integer

References

• bcl2fastq source code from illumina downloads https://support.illumina.com/sequencing/sequencing_software/bcl2fastq-conversion-software/downloads.html

• Spec file from illumina support https://support.illumina.com/content/dam/illumina-support/documents/documentation/software_documentation/bcl2fastq/bcl2fastq_letterbooklet_15038058brpmi.pdf

• http://support-docs.illumina.com/IN/NovaSeq6000Dx_HTML/Content/IN/NovaSeq/SequencingOutputFiles_fNV.htm

• https://help.basespace.illumina.com/files-used-by-basespace/fastq-files

• https://docs.python.org/3/library/struct.html#format-characters

See also mkdata.sh file in bcl2fastq source code for insights on bcl format.