Four genes, (ESR1, CCND1, CHUK and IGF-1) may be critical in mammary gland development: ESR1 was regulated by most miRNAs, CCND1 was involved in 17 pathways, CHUK and IGF-1 were involved in 14 and 12 pathways, respectively, and three pathways (Ko05200: pathway in cancer, Ko05520: chronic myeloid leukaemia and Ko04630: Jak-STAT signalling pathway) may be valuable in mammary gland development; the numbers of genes involved in each pathway were seven, five and five, respectively. This evidence concerns the gene CCND1 and cancer.