Go here --->
http://www.ncbi.nlm.nih.gov/BLAST/ Click on the type of query you would like to do. Then put in the sequence you would like to check. After the process is completed, a listing of matches found in the searched databases will be given to you. In that data is a number describing the probability of finding a random sequence in the database. Here is one for a 300 base string.
The probability is 10-167 with 0 mutations.
>gi|5729841|ref|NM_006708.1| 
Homo sapiens glyoxalase I (GLO1), mRNA
Length = 1993
Score = 595 bits (300), Expect = e-167
Identities = 300/300 (100%)
Strand = Plus / Plus
Query: 1 ctagttaaggcggcacagggccgaggcgtagtgtgggtgactcctccgttccttgggtcc 60
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sbjct: 1 ctagttaaggcggcacagggccgaggcgtagtgtgggtgactcctccgttccttgggtcc 60
Query: 61 cgtcgtctgtgatactgcagttcagccatggcagaaccgcagcccccgtccggcggcctc 120
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sbjct: 61 cgtcgtctgtgatactgcagttcagccatggcagaaccgcagcccccgtccggcggcctc 120
Query: 121 acggacgaggccgccctcagttgctgctccgacgcggaccccagtaccaaggattttcta 180
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sbjct: 121 acggacgaggccgccctcagttgctgctccgacgcggaccccagtaccaaggattttcta 180
Query: 181 ttgcagcagaccatgctacgagtgaaggatcctaagaagtcactggatttttatactaga 240
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sbjct: 181 ttgcagcagaccatgctacgagtgaaggatcctaagaagtcactggatttttatactaga 240
Query: 241 gttcttggaatgacgctaatccaaaaatgtgattttcccattatgaagttttcactctac 300
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sbjct: 241 gttcttggaatgacgctaatccaaaaatgtgattttcccattatgaagttttcactctac 300
Here is the result for the mouse compared to human. 10-26 with 27 mutations.
>gi|26327652|dbj|AK031832.1| Mus musculus adult male medulla oblongata cDNA, RIKEN full-length
enriched library, clone:6330414G20 product:GLYOXALASE I
homolog [Homo sapiens], full insert sequence
Length = 959
Score = 127 bits (64), Expect = 1e-26
Identities = 145/172 (84%)
Strand = Plus / Plus
Query: 83 cagccatggcagaaccgcagcccccgtccggcggcctcacggacgaggccgccctcagtt 142
||||||||||||| || ||||| ||||| | |||||||| || ||| |||| |||| |
Sbjct: 46 cagccatggcagagccacagccggcgtccagtggcctcactgatgagaccgctttcagct 105
Query: 143 gctgctccgacgcggaccccagtaccaaggattttctattgcagcagaccatgctacgag 202
|||||||||| | ||||| || ||||||||||||||| ||||||| || |||||| ||
Sbjct: 106 gctgctccgatccagaccctagcaccaaggattttctactgcagcaaacgatgctaagaa 165
Query: 203 tgaaggatcctaagaagtcactggatttttatactagagttcttggaatgac 254
| ||||||||||||||||| |||||||||||||| || ||||||||| ||||
Sbjct: 166 ttaaggatcctaagaagtccctggatttttatacgagggttcttggactgac 217