In mammals, the large IGF-I gene comprises 6 exons, which are subject to alternative splicing. All transcripts contain exons 3 and 4, encoding mature IGF-I, but the other exons are included in various combinations, giving at least 6 possible mature mRNAs. At the 5′ end, exons 1 and 2 are spliced alternatively to exon 3, giving different leader/signal sequences. It is shown in this study that in human an additional exon (designated exon 0) is present, upstream of exon 1. This can be spliced directly to exon 3 or, less frequently, into exon 1. Exon 0 is utilized in liver, in about 24% of IGF-I transcripts, to a minor extent in prostate and endometrium (<1% of transcripts), but not in any of 29 other normal human tissues examined. The exon 0 sequence includes an in-frame ATG/AUG, potentially providing a translation start point giving an IGF-I precursor with a very long signal peptide. However, this ATG is very close to the 5′ end, and may not be included in all transcripts; an in-frame ATG in exon 3 could provide an alternative start point. Utilization of exon 0 was detected in other apes, and to a small extent in Old World monkeys, but not in New World monkeys, prosimians or various non-primate mammals. Exon 0 was not expressed in most human tumours, but was utilized in many prostate tumours, at levels much greater than seen in normal prostate, and in liver tumours, at a lower level than in normal liver.

在哺乳动物中，大的IGF-I基因包含6个外显子，可以进行选择性剪接。所有的转录本均含有编码成熟IGF-1的外显子3和4，但其他外显子以各种组合形式包含在内，从而至少提供了6种可能的成熟mRNA。在5'端，外显子1和2与外显子3交替剪接，给出了不同的前导/信号序列。在这项研究中表明，在人类中，在外显子1的上游还存在一个额外的外显子（称为外显子0）。该外显子可以直接剪接到外显子3上，或者更不常见地剪接到外显子1上。外显子0用于肝脏，大约24％的IGF-I转录本在前列腺和子宫内膜中占很小的比例（<1％的转录本），但在其他29个正常人体组织中却没有。外显子0序列包括帧内ATG / AUG，可能提供翻译起点，从而为IGF-I前体提供非常长的信号肽。但是，该ATG非常靠近5'末端，可能未包含在所有转录本中；外显子3中的帧内ATG可以提供替代的起点。在其他猿类中检出了外显子0的利用，并在很小的程度上在旧世界的猴子中发现了，但在新世界的猴子，prosimians或各种非灵长类哺乳动物中却没有发现。外显子0在大多数人类肿瘤中均未表达，但在许多前列腺肿瘤中使用的水平远高于正常前列腺中所见的水平，而在肝肿瘤中使用的水平低于正常肝脏中的水平。在其他猿类中检出了外显子0的利用，并在很小的程度上在旧世界的猴子中发现了，但在新世界的猴子，prosimians或各种非灵长类哺乳动物中却没有发现。外显子0在大多数人类肿瘤中均未表达，但在许多前列腺肿瘤中使用的水平远高于正常前列腺中所见的水平，而在肝肿瘤中使用的水平低于正常肝脏中的水平。在其他猿类中检出了外显子0的利用，并在很小的程度上在旧世界的猴子中发现了，但在新世界的猴子，prosimians或各种非灵长类哺乳动物中却没有发现。外显子0在大多数人类肿瘤中均未表达，但在许多前列腺肿瘤中使用的水平远高于正常前列腺中所见的水平，而在肝肿瘤中使用的水平低于正常肝脏中的水平。