Genomic structure of human SPAM1 . (A) Overview of the SPAM1 locus. Exons are boxed and numbered, with the size in bp shown above. Intron sizes in kb are shown below. The approximate positions of the SPAM1 ORF start and stop codons are indicated. The four SPAM1 splicing variants are shown schematically below. ERV-derived sequences are shown in green, with other sequences in blue. The diagram is not to scale. A black bar indicates the region shown in more detail in Figure 1B and 2. (B) The positions of SPAM1 alternative first exons 1A and 1B with respect to TE sequences. Exons are represented by solid black boxes. Bent arrows indicate the position and relative usage of the 5'-most transcription start site for each exon. TEs are represented by the colored boxes below; arrows indicate the orientation of each TE.