Hereditary nonpolyposis colon cancer results from heritable defects in the MLH1, MSH2, PMS1 and PMS2 genes, which encode proteins involved in the mismatch repair process. In this work we report the identification of two novel germline mutations in the MLH1 gene from two unrelated HNPCC families. The two affected families do not fulfill the Amsterdam criteria. In family 1 we found a missense S93G mutation, which lies in a MLH1 domain critical for its MMR functions. In family 2 we found a two nucleotide insertion (AG) in position 523 from the AUG which determines an early stop codon at position 606 (codon 203). In both families the mutant alleles cosegregate with the cancer phenotype.