Genesis 6:1
Now it came about, when mankind began to multiply on the face of the land, and daughters were born to them, 2 that the sons of God saw that the daughters of mankind were beautiful; and they took wives for themselves, whomever they chose. 3 Then the LORD said, “My Spirit will not remain with man forever, because he is also flesh; nevertheless his days shall be 120 years.” 4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of mankind, and they bore children to them. Those were the mighty men who were of old, men of renown.
5 Then the LORD saw that the wickedness of mankind was great on the earth, and that every intent of the thoughts of their hearts was only evil continually. 6 So the LORD was sorry that He had made mankind on the earth, and He was grieved in His heart. 7 Then the LORD said, “I will wipe out mankind whom I have created from the face of the land; mankind, and animals as well, and crawling things, and the birds of the sky. For I am sorry that I have made them.” 8 But Noah found favor in the eyes of the LORD.
My interpretation of much of early Genesis scripture is rather different than dogma of millenia of ignorant religious authorities including that pertaining to Nephilim. Instead, the so-called Nephilim read like genetically altered Adam and Eve descendants. In fact, scripture per above states they were men. And further, "mighty" doesn't necessarily imply "giants" as in ridiculous popular lore. One needs to understand, any extraterrestrial entities that are also DNA based would never be able to mate successfully with homo sapiens or for that matter, ANY mammals etc, because of the vast trail of different varying from earliest evolutionary genetic sequences on any specific planet would be impossibly different. So all those science fiction movies where some alien mates with humans is utter impossible nonsense scientifically.